Hollow Structured Silicon Anodes with Stabilized Solid Electrolyte Interphase Film for Lithium-Ion Batteries

Exploration of High and Low Molecular Weight Polyacrylic Acids and Sodium Polyacrylates as Potential Binder System for Use in Silicon Graphite Anodes

The commercialization of silicon anodes requires polymer binders that are both mechanically robust and electrochemically stable in order to ensure that they can accommodate the volume expansion experienced during cycling. In this study, we examine the use of both low and high molecular weight (MW) polyacrylic acids (PAAs), and sodium polyacrylates (Na-PAAs), at different degrees of partial neutralization, as a possible binder candidate for use in silicon graphite anodes. High MW PAAs were found to have stable capacity retentions of 672 mAh g-1 for over 100 cycles, whereas with the low MW PAAs the capacity was found to already have declined to 373 mAh g-1 after the first 30 cycles. Furthermore, the partial neutralization of Na-PAA binder system was found to provide superior cycling performances, as compared to non-neutralized or fully neutralized PAA systems. The high MW and partially neutralized PAAs were also found to provide the electrode coatings with higher cohesion strengths, which allow for the electrodes' microstructure to be more effectively maintained over several cycles. Overall, these findings suggest that partially neutralized and higher MW PAAs are the more suitable polymer binder candidates for use within silicon-graphite anodes.

Reduced Volume Expansion of Micron-Sized SiOx via Closed-Nanopore Structure Constructed by Mg-Induced Elemental Segregation

The inherently huge volume expansion during Li uptake has hindered the use of Si-based anodes in high-energy lithium-ion batteries. While some pore-forming and nano-architecting strategies show promises to effectively buffer the volume change, other parameters essential for practical electrode fabrication, such as compaction density, are often compromised. Here we propose a new in situ Mg doping strategy to form closed-nanopore structure into a micron-sized SiOx particle at a high bulk density. The doped Mg atoms promote the segregation of O, so that high-density magnesium silicates form to generate closed nanopores. By altering the mass content of Mg dopant, the average radii (ranged from 5.4 to 9.7 nm) and porosities (ranged from 1.4 % to 15.9 %) of the closed pores are precisely adjustable, which accounts for volume expansion of SiOx from 77.8 % to 22.2 % at the minimum. Benefited from the small volume variation, the Mg-doped micron-SiOx anode demonstrates improved Li storage performance towards realization of a 700-(dis)charge-cycle, 11-Ah-pouch-type cell at a capacity retention of >80 %. This work offers insights into reasonable design of the internal structure of micron-sized SiOx and other materials that undergo conversion or alloying reactions with drastic volume change, to enable high-energy batteries with stable electrochemistry.

Designing superior solid electrolyte interfaces on silicon anodes for high-performance lithium-ion batteries

The solid electrolyte interface (SEI) is a passivation layer formed on the surface of lithium-ion battery (LIB) anode materials produced by electrolyte decomposition. The quality of the SEI plays a critical role in the cyclability, rate capacity, irreversible capacity loss and safety of lithium-ion batteries (LIBs). The stability of the SEI is especially important for Si anodes which experience tremendous volume changes during cycling. Therefore, in this review we discuss the effect of the SEI on Si anodes. Firstly, the mechanism of formation, composition, and component properties of solid electrolyte interfaces (SEIs) are introduced, and the SEI of native-oxide-terminated Si is emphasized. Then the growth model and mechanical failure of SEIs are analyzed in detail, and the challenges facing SEIs of Si anodes are proposed. Moreover, we highlight several modification methods for SEIs on Si anodes, including electrolyte additives, surface-functionalization of Si, coating artificial SEIs or protective layers, and the structural design of Si-based composites. We believe that designing a high-quality SEI is of great significance and is beneficial for the improved electrochemical performance of Si anodes.

Quantification on Growing Mass of Solid Electrolyte Interphase and Deposited Mn(II) on the Silicon Anode of LiMn2O4 Full Lithium-Ion Cells

Silicon is considered to be one of the most important high-energy density anode materials for next-generation lithium-ion batteries. A large number of experimental studies on silicon anode have achieved better results, and greatly promoted its practical application potentiality, but almost of them are only tested in metal lithium half batteries. There is still an unavoidable question for commercial applications: what is the performance of the full cell composed of a silicon anode and a manganese-based material cathode? In this paper, the growing solid electrolyte interphase (SEI) and deposited manganese ions of the silicon anode's surface of the spinel lithium manganese oxide LiMn₂O₄/silicon full cells are quantitatively studied during electrochemical cycling, and the SEI performances are tested by differential scanning calorimetry to find out the reason for the rapid decline of reversible capacity in the LiMn₂O₄/silicon system. The experimental results show that manganese ions can make SEI films rapidly grow on the silicon anode and make SEI films more brittle, which results in lower Coulombic efficiency and rapid decline in capacity of the silicon anode.

Lithiation Behavior of Coaxial Hollow Nanocables of Carbon-Silicon Composite

A design of coaxial hollow nanocables of carbon nanotubes and silicon composite (CNTs@Silicon) was presented, and the lithiation/delithiation behavior was investigated. The FIB-SEM studies demonstrated hollow structured silicon tends to expand inward and shrink outward during lithiation/delithiation, which reveal the mechanism of inhibitive effect of the excessive growth of solid-electrolyte interface by hollow structured silicon. The as-prepared coaxial hollow nanocables demonstrate an impressive reversible specific capacity of 1150 mAh g^-1 over 500 cycles, giving an average Coulombic efficiency of >99.9%. The electrochemical impedance spectroscopy and differential scanning calorimetry confirmed the SEI film excessive growth is prevented.

Nanocomposite of Si/C Anode Material Prepared by Hybrid Process of High-Energy Mechanical Milling and Carbonization for Li-Ion Secondary Batteries

Si/C nanocomposite was successfully prepared by a scalable approach through high-energy mechanical milling and carbonization process. The crystalline structure of the milled powders was studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Morphology of the milled powders was investigated by Field-emission scanning electron microscopy (FE-SEM). The effects of milling time on crystalline size, crystal structure and microstructure, and the electrochemical properties of the nanocomposite powders were studied. The nanocomposite showed high reversible capacity of ~1658 mAh/g with an initial cycle coulombic efficiency of ~77.5%. The significant improvement in cyclability and the discharge capacity was mainly ascribed to the silicon particle size reduction and carbon layer formation over silicon for good electronic conductivity. As the prepared nanocomposite Si/C electrode exhibits remarkable electrochemical performance, it is potentially applied as a high capacity anode material in the lithium-ion secondary batteries.

Exploring Critical Factors Affecting Strain Distribution in 1D Silicon-Based Nanostructures for Lithium-Ion Battery Anodes

Despite the advantage of high capacity, the practical use of the silicon anode is still hindered by large volume expansion during the severe pulverization lithiation process, which results in electrical contact loss and rapid capacity fading. Here, a combined electrochemical and computational study on the factor for accommodating volume expansion of silicon-based anodes is shown. 1D silicon-based nanostructures with different internal spaces to explore the effect of spatial ratio of voids and their distribution degree inside the fibers on structural stability are designed. Notably, lotus-root-type silicon nanowires with locally distributed void spaces can improve capacity retention and structural integrity with minimum silicon pulverization during lithium insertion and extraction. The findings of this study indicate that the distribution of buffer spaces, electrochemical surface area, as well as Li diffusion property significantly influence cycle performance and rate capability of the battery, which can be extended to other silicon-based anodes to overcome large volume expansion.

Pulverization Control by Confining Fe3O4 Nanoparticles Individually into Macropores of Hollow Carbon Spheres for High-Performance Li-Ion Batteries

In this article, double carbon shell hollow spheres which provide macropores (mC) for ultrasmall Fe₃O₄ nanoparticle (10-20 nm) encapsulation individually were first prepared (Fe₃O₄@mC). The well-constructed Fe₃O₄@mC electrode materials offer the feasibility to study the volume change, aggregation, and pulverization process of the active Fe₃O₄ nanoparticles for Li-ion storage in a confined space. Fe₃O₄@mC exhibits excellent electrochemical performances and delivers a high capacity of 645 mA h g^-1 at 2 A g^-1 after 1000 cycles. Even at 10 A g^-1 or after 1000 cycles at 2 A g^-1, the porous carbon structure was well maintained and no obvious aggregation and pulverization of the Fe₃O₄ nanoparticles was observed, although the volume of the active Fe₃O₄ particles was expanded to 40-60 nm compared to that of the original particles (10-20 nm). This can be due to the in situ embedment of one Fe₃O₄ nanoparticle into one macropore individually. The uniform dispersion and confinement of the Fe₃O₄ nanoparticles in the macropores of the carbon shell could effectively accommodate severe volume variations upon cycling and prevent self-aggregation and spreading out from the carbon shell during the expansion process of the nanoscale Fe₃O₄ particles, leading to improved capacity retention. Our work confirms the effectiveness for pulverization control by confining Fe₃O₄ nanoparticles individually into macropores to improve its Li-ion storage properties, providing a novel strategy for the design of new-structured anode materials for Li-ion batteries.

Tuning Porosity and Surface Area in Mesoporous Silicon for Application in Li-Ion Battery Electrodes

This work aims to improve the poor cycle lifetime of silicon-based anodes for Li-ion batteries by tuning microstructural parameters such as pore size, pore volume, and specific surface area in chemically synthesized mesoporous silicon. Here we have specifically produced two different mesoporous silicon samples from the magnesiothermic reduction of ordered mesoporous silica in either argon or forming gas. In situ X-ray diffraction studies indicate that samples made in Ar proceed through a Mg₂Si intermediate, and this results in samples with larger pores (diameter ≈ 90 nm), modest total porosity (34%), and modest specific surface area (50 m² g^-1). Reduction in forming gas, by contrast, results in direct conversion of silica to silicon, and this produces samples with smaller pores (diameter ≈ 40 nm), higher porosity (41%), and a larger specific surface area (70 m² g^-1). The material with smaller pores outperforms the one with larger pores, delivering a capacity of 1121 mAh g^-1 at 10 A g^-1 and retains 1292 mAh g^-1 at 5 A g^-1 after 500 cycles. For comparison, the sample with larger pores delivers a capacity of 731 mAh g^-1 at 10 A g^-1 and retains 845 mAh g^-1 at 5 A g^-1 after 500 cycles. The dependence of capacity retention and charge storage kinetics on the nanoscale architecture clearly suggests that these microstructural parameters significantly impact the performance of mesoporous alloy type anodes. Our work is therefore expected to contribute to the design and synthesis of optimal mesoporous architectures for advanced Li-ion battery anodes.

Study of Lithium Silicide Nanoparticles as Anode Materials for Advanced Lithium Ion Batteries

The development of high-performance silicon anodes for the next generation of lithium ion batteries (LIBs) evokes increasing interest in studying its lithiated counterpart-lithium silicide (Li_xSi). In this paper we report a systematic study of three thermodynamically stable phases of Li_xSi (x = 4.4, 3.75, and 2.33) plus nitride-protected Li_4.4Si, which are synthesized via the high-energy ball-milling technique. All three Li_xSi phases show improved performance over that of unmodified Si, where Li_4.4Si demonstrates optimum performance with a discharging capacity of 3306 (mA h)/g initially and maintains above 2100 (mA h)/g for over 30 cycles and above 1200 (mA h)/g for over 60 cycles at the current density of 358 mA/g of Si. A fundamental question studied is whether different electrochemical paradigms, that is, delithiation first or lithiation first, influence the electrode performance. No significant difference in electrode performance is observed. When a nitride layer (Li_xN_ySi_z) is created on the surface of Li_4.4Si, the cyclability is improved to retain the capacity above 1200 (mA h)/g for more than 80 cycles. By increasing the nitridation extent, the capacity retention is improved significantly from the average decrease of 1.06% per cycle to 0.15% per cycle, while the initial discharge capacity decreases due to the inactivity of Si in the Li_xN_ySi_z layer. Moreover, the Coulombic efficiencies of all Li_xSi-based electrodes in the first cycle are significantly higher than that of a Si electrode (∼90% vs 40-70%).

Confined Solid Electrolyte Interphase Growth Space with Solid Polymer Electrolyte in Hollow Structured Silicon Anode for Li-Ion Batteries

Silicon anodes for lithium-ion batteries are of much interest owing to their extremely high specific capacity but still face some challenges, especially the tremendous volume change which occurs in cycling and further leads to the disintegration of electrode structure and excessive growth of solid electrolyte interphase (SEI). Here, we designed a novel approach to confine the inward growth of SEI by filling solid polymer electrolyte (SPE) into pores of hollow silicon spheres. The as-prepared composite delivers a high specific capacity of more than 2100 mAh g^-1 and a long-term cycle stability with a reversible capacity of 1350 mAh g^-1 over 500 cycles. The growing behavior of SEI was investigated by electrochemical impedance spectroscopy and differential scanning calorimetry, and the results revealed that SPE occupies the major space of SEI growth and thus confines its excessive growth, which significantly improves cycle performance and Coulombic efficiency of cells embracing hollow silicon spheres.

Hierarchical Mesoporous Iron Fluoride with Superior Rate Performance for Lithium-Ion Batteries

Monodispersed mesoporous iron fluorides were synthesized by a low-cost reversed micelle method. The as-prepared materials with hierarchical mesoporous structure exhibit excellent rate capability (115.6 mAh g^-1 at 2000 mA g^-1) which is superior to many other carbon-free iron fluorides. In addition, a high reversible capacity of 143.2 mAh g^-1 can be retained after 100 cycles at 1000 mA g^-1. The outstanding electrochemical features can be attributed to the particular hierarchical mesoporous structure, facilitating electrolyte penetration as well as rapid electronic and ionic transportation.

Constructing Novel Si@SnO2 Core-Shell Heterostructures by Facile Self-Assembly of SnO2 Nanowires on Silicon Hollow Nanospheres for Large, Reversible Lithium Storage

Developing an industrially viable silicon anode, featured by the highest theoretical capacity (4200 mA h g(-1)) among common electrode materials, is still a huge challenge because of its large volume expansion during repeated lithiation-delithiation as well as low intrinsic conductivity. Here, we expect to address these inherent deficiencies simultaneously with an interesting hybridization design. A facile self-assembly approach is proposed to decorate silicon hollow nanospheres with SnO2 nanowires. The two building blocks, hand in hand, play a wonderful duet by bridging their appealing functionalities in a complementary way: (1) The silicon hollow nanospheres, in addition to the major role as a superior capacity contributor, also act as a host material (core) to partially accommodate the volume expansion, thus alleviating the capacity fading by providing abundant hollow interiors, void spaces, and surface areas. (2) The SnO2 nanowires serve as a conductive coating (shell) to enable efficient electron transport due to a relatively high conductivity, thereby improving the cyclability of silicon. Compared to other conductive dopants, the SnO2 nanowires with a high theoretical capacity (790 mA h g(-1)) can contribute outstanding electrochemical reaction kinetics, further adding value to the ultimate electrochemical performances. The resulting novel Si@SnO2 core-shell heterostructures exhibit remarkable synergy in large, reversible lithium storage, delivering a reversible capacity as high as 1869 mA h g(-1)@500 mA g(-1) after 100 charging-discharging cycles.

NiSi(x)/a-Si Nanowires with Interfacial a-Ge as Anodes for High-Rate Lithium-Ion Batteries

Conductive metal nanowire is a promising current collector for the Si-based anode material in high-rate lithium-ion batteries. However, to harness this remarkable potential for high power density energy storage, one has to address the interfacial potential barrier that hinders the electron injection from the metal side. Herein, we present that, solely by inserting ultrathin amorphous germanium (a-Ge) (∼5 nm) at the interface of NiSix/amorphous Si (a-Si), the rate capacity was substantially enhanced, 477 mAh g(-1) even at a high rate of 40 A g(-1). In addition, batteries containing the NiSix/Ge+Si anodes cycled over 1000 times at 10 A g(-1) while the capacity retaining more than 877 mAh g(-1), which is among the highest reported. The excellent electrochemical performance is directly correlated with the significantly improved electrical conductivity and mechanical stability throughout the entire electrode. The potential barrier between the NiSix and a-Si was modulated by a-Ge, which constructs an electron highway. Besides, the a-Ge interlayer enhances the interfacial adhesion by reducing void fraction and the inhomogeneous strain of the Li-Ge and Li-Si stacking structure was accommodated through the bending and twist of relatively thin NiSix, thus ensures a more stable high-rate cycling performance. Our work shows an effective way to fabricate metal/a-Si nanowires for high-rate lithium-ion battery anodes.