Solid Electrolyte Interphase Growth and Capacity Loss in Silicon Electrodes

Electrochemically Induced Amorphization and Unique Lithium and Sodium Storage Pathways in FeSbO4 Nanocrystals

The increasing energy demands have prompted research on conversion and alloying materials, offering high lithium and sodium storage capacities. However, most of these materials suffer from huge volume expansion and degradation over the thousands of charging and discharging cycles required for commercial applications. In this study, we demonstrate a facile route to synthesize FeSbO₄ nanocrystals that possess theoretical lithium and sodium storage capacity of 1220 mAh g^-1. Operando X-ray diffraction studies reveal the electrochemically induced amorphization of the nanocrystals upon alkali-ion storage. We achieved specific storage capacities of ∼600 mAh g^-1 for lithium and ∼300 mAh g^-1 for sodium, respectively. The disparity in the lithium and sodium electrochemistry arises from the unique lithiation/sodiation pathways adopted by the nanocrystals. This study offers new insights into the chemistry and mechanism of conversion- and alloying-based energy storage materials that would greatly assist the development of next-generation active materials for energy storage.

High Volumetric and Gravimetric Capacity Electrodeposited Mesostructured Sb2 O3 Sodium Ion Battery Anodes

Sodium ion batteries (SIBs) are considered promising alternatives to lithium ion batteries for grid-scale and other energy storage applications because of the broad geographical distribution and low cost of sodium relative to lithium. Here, fabrication and characterization of high gravimetric and volumetric capacity 3D Ni-supported Sb₂ O₃ anodes for SIBs are presented. The electrodes are prepared by colloidal templating and pulsed electrodeposition followed by heat treatment. The colloidal template is optimized to provide large pore interconnects in the 3D scaffold to enable a high active materials loading and accommodate a large volume expansion during cycling. An electrodeposited loading of 1.1 g cm^-3 is chosen to enable a combined high gravimetric and volumetric capacity. At this loading, the electrodes exhibit a specific capacity of ≈445 mA h g^-1 and a volumetric capacity of ≈488 mA h cm^-3 with a capacity retention of 89% after 200 cycles at 200 mA g^-1 . The stable cycling performance can be attributed to the 3D metal scaffold, which supports active materials undergoing large volume changes, and an initial heat treatment appears to improve the adhesion of the Sb₂ O₃ to the metal scaffold.

Si nanoflake-assembled blocks towards high initial coulombic efficiency anodes for lithium-ion batteries

Assisted by artificial amorphous copper silicate, Si with a flake-like structure was obtained through a facile magnesiothermic reduction. The Si anodes exhibit excellent cyclic performance and rate performance. Particularly, a high initial coulombic efficiency of 85%-89% was obtained due to their greatly reduced surface and internal defects.

What Can We Learn from Solid State NMR on the Electrode-Electrolyte Interface?

Rechargeable battery cells are composed of two electrodes separated by an ion-conducting electrolyte. While the energy density of the cell is mostly determined by the redox potential of the electrodes and amount of charge they can store, the processes at the electrode-electrolyte interface govern the battery's lifetime and performance. Viable battery cells rely on unimpeded ion transport across this interface, which depends on its composition and structure. These properties are challenging to determine as interfacial phases are thin, disordered, heterogeneous, and can be very reactive. The recent developments and applications of solid state NMR spectroscopy in the study of interfacial phenomena in rechargeable batteries based on lithium and sodium chemistries are reviewed. The different NMR interactions are surveyed and how these are used to shed light on the chemical composition and architecture of interfacial phases as well as directly probe ion transport across them is described. By combining new methods in solid state NMR spectroscopy with other analytical tools, a holistic description of the electrode-electrolyte interface can be obtained. This will enable the design of improved interfaces for developing battery cells with high energy, high power, and longer lifetime.

In situ synthesis of porous Si dispersed in carbon nanotube intertwined expanded graphite for high-energy lithium-ion batteries

Silicon (Si) is perceived as one of the most promising anode materials for next-generation lithium-ion batteries (LIBs). For its practical application, superior electrochemical properties, low cost and scalable production are highly required. Herein, we synthesize a carbon nanotube intertwined expanded graphite/porous Si (CNT/EG/pSi) composite through the in situ magnesiothermic reduction method, where porous Si nanoparticles (NPs) are dispersed in the interspaces constructed by EG sheets, with CNTs intertwined throughout the composite, connecting Si NPs and EG sheets. Mesopores within Si NPs can not only shorten the electron and Li+ ion transport distance, but also play an important role in accommodating the huge volume change. EG and CNTs construct a three-dimensional conductive network, improving the electronic conductivity of the composite. Moreover, EG sheets release the excessive local stress over cycles, and CNTs can randomly build new electronic pathways as the structure changes, alleviating the degeneration of the conductive network. Consequently, the CNT/EG/pSi composite exhibits enhanced cycling and rate performances when used as the anode material, delivering reversible specific capacities of 2618 mA h g-1 at 0.2 A g-1 and 1390 mA h g-1 at 4 A g-1, maintaining a capacity of 2152 mA h g-1 after 100 cycles at 0.4 A g-1, with a capacity retention of 84%. This hierarchically structured anode material has a facile and low-cost synthetic route, as well as excellent electrochemical performances, making it attractive for high-performance LIB applications.

Understanding Fluoroethylene Carbonate and Vinylene Carbonate Based Electrolytes for Si Anodes in Lithium Ion Batteries with NMR Spectroscopy

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are widely used as electrolyte additives in lithium ion batteries. Here we analyze the solid electrolyte interphase (SEI) formed on binder-free silicon nanowire (SiNW) electrodes in pure FEC or VC electrolytes containing 1 M LiPF₆ by solid-state NMR with and without dynamic nuclear polarization (DNP) enhancement. We find that the polymeric SEIs formed in pure FEC or VC electrolytes consist mainly of cross-linked poly(ethylene oxide) (PEO) and aliphatic chain functionalities along with additional carbonate and carboxylate species. The formation of branched fragments is further confirmed by ¹³C-¹³C correlation NMR experiments. The presence of cross-linked PEO-type polymers in FEC and VC correlates with good capacity retention and high Coulombic efficiencies of the SiNWs. Using ²⁹Si DNP NMR, we are able to probe the interfacial region between SEI and the Si surface for the first time with NMR spectroscopy. Organosiloxanes form upon cycling, confirming that some of the organic SEI is covalently bonded to the Si surface. We suggest that both the polymeric structure of the SEI and the nature of its adhesion to the redox-active materials are important for electrochemical performance.

The Electrochemical Performance of Silicon Nanoparticles in Concentrated Electrolyte

Silicon is a promising material for anodes in energy-storage devices. However, excessive growth of a solid-electrolyte interphase (SEI) caused by the severe volume change during the (de)lithiation processes leads to dramatic capacity fading. Here, we report a super-concentrated electrolyte composed of lithium bis(fluorosulfonyl)imide (LiFSI) and propylene carbonate (PC) with a molar ratio of 1:2 to improve the cycling performance of silicon nanoparticles (SiNPs). The SiNP electrode shows a remarkably improved cycling performance with an initial delithiation capacity of approximately 3000 mAh g^-1 and a capacity of approximately 2000 mAh g^-1 after 100 cycles, exhibiting about 6.8 times higher capacity than the cells with dilute electrolyte LiFSI-(PC)₈ . Raman spectra reveal that most of the PC solvent and FSI anions are complexed by Li⁺ to form a specific solution structure like a fluid polymeric network. The reduction of FSI anions starts to play an important role owing to the increased concentration of contact ion pairs (CIPs) or aggregates (AGGs), which contribute to the formation of a more mechanically robust and chemically stable complex SEI layer. The complex SEI layer can effectively suppress the morphology evolution of silicon particles and self-limit the excessive growth, which mitigates the crack propagation of the silicon electrode and the deterioration of the kinetics. This study will provide a new direction for screening cycling-stable electrolytes for silicon-based electrodes.

The Chemical Structure of Carbon Nanothreads Analyzed by Advanced Solid-State NMR

Carbon nanothreads are a new type of one-dimensional sp³-carbon nanomaterial formed by slow compression and decompression of benzene. We report characterization of the chemical structure of ¹³C-enriched nanothreads by advanced quantitative, selective, and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments complemented by infrared (IR) spectroscopy. The width of the NMR spectral peaks suggests that the nanothread reaction products are much more organized than amorphous carbon. In addition, there is no evidence from NMR of a second phase such as amorphous mixed sp²/sp³-carbon. Spectral editing reveals that almost all carbon atoms are bonded to one hydrogen atom, unlike in amorphous carbon but as is expected for enumerated nanothread structures. Characterization of the local bonding structure confirms the presence of pure fully saturated "degree-6" carbon nanothreads previously deduced on the basis of crystal packing considerations from diffraction and transmission electron microscopy. These fully saturated threads comprise between 20% and 45% of the sample. Furthermore, ¹³C-¹³C spin exchange experiments indicate that the length of the fully saturated regions of the threads exceeds 2.5 nm. Two-dimensional ¹³C-¹³C NMR spectra showing bonding between chemically nonequivalent sites rule out enumerated single-site thread structures such as polytwistane or tube (3,0) but are consistent with multisite degree-6 nanothreads. Approximately a third of the carbon is in "degree-4" nanothreads with isolated double bonds. The presence of doubly unsaturated degree-2 benzene polymers can be ruled out on the basis of ¹³C-¹³C NMR with spin exchange rate constants tuned by rotational resonance and ¹H decoupling. A small fraction of the sample consists of aromatic rings within the threads that link sections with mostly saturated bonding. NMR provides the detailed bonding information necessary to refine solid-state organic synthesis techniques to produce pure degree-6 or degree-4 carbon nanothreads.

Capacity fade in high energy silicon-graphite electrodes for lithium-ion batteries

A silicon-graphite blended anode is paired with a high capacity LiFePO4 reference/counter electrode to track irreversibility and lithium inventory. The LiFePO4 electrode provides a reliable, flat potential for dQ dV-1 analysis of LixSi and LixC electrochemical reactions. We relate this electrochemistry to the morphological and physical changes taking place.

Evolving affinity between Coulombic reversibility and hysteretic phase transformations in nano-structured silicon-based lithium-ion batteries

Nano-structured silicon is an attractive alternative anode material to conventional graphite in lithium-ion batteries. However, the anode designs with higher silicon concentrations remain to be commercialized despite recent remarkable progress. One of the most critical issues is the fundamental understanding of the lithium-silicon Coulombic efficiency. Particularly, this is the key to resolve subtle yet accumulatively significant alterations of Coulombic efficiency by various paths of lithium-silicon processes over cycles. Here, we provide quantitative and qualitative insight into how the irreversible behaviors are altered by the processes under amorphous volume changes and hysteretic amorphous-crystalline phase transformations. Repeated latter transformations over cycles, typically featured as a degradation factor, can govern the reversibility behaviors, improving the irreversibility and eventually minimizing cumulative irreversible lithium consumption. This is clearly different from repeated amorphous volume changes with different lithiation depths. The mechanism behind the correlations is elucidated by electrochemical and structural probing.

A step towards understanding the beneficial influence of a LIPON-based artificial SEI on silicon thin film anodes in lithium-ion batteries

In this work, we present a comprehensive study on the influence of lithium phosphorus oxynitride (LIPON) as a possible "artificial SEI layer" on the electrochemical performance of pure silicon (Si) thin film electrodes for a possible application in microbatteries or on-chip batteries. Si thin film anodes (140 nm) with and without an additional amorphous LIPON surface layer of different thicknesses (100-300 nm) were prepared by magnetron sputter deposition. The LIPON surface coating was characterized thoroughly by means of electrochemical impedance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. In situ electrochemical dilatometry and ex situ cross-section analysis of the electrodes after cycling could prove that the LIPON coating greatly diminishes the volume expansion of the Si electrode and, therefore, significantly improves the cycling stability and capacity retention. Furthermore, the LIPON coating remarkably reduces parasitic electrolyte decomposition reactions that originate from the Si volume expansion and contribute to the overall electrode volume expansion, as observed by the enhanced Coulombic efficiency over ongoing charge/discharge cycling. Overall, this article focuses on the preparation of optimized Si-based thin film electrodes in combination with LIPON solid electrolyte coatings for use in high-energy lithium ion batteries.

Side-by-side observation of the interfacial improvement of vertical graphene-coated silicon nanocone anodes for lithium-ion batteries by patterning technology

We report that vertical graphene coating can greatly improve the electrochemical performance and the interfacial stability of silicon nanocone (SNC) anodes for lithium-ion batteries. The coating patterning technology is innovatively employed for side-by-side demonstration of the exclusive influences of graphene coating on the solid-electrolyte interphase (SEI) formation and the structural stability of the SNC electrode. The silicon nanocone-graphene (SNC-G) electrode achieves a longer cycle life (1715 cycles), higher Coulombic efficiency (average 98.2%), better rate capability, and lower electrode polarization than the SNC electrode. The patterning of the graphene coating provides a much direct and convincing morphological comparison between the SNC-G structure and the SNC structure, showing clearly that the SNC-G area maintains a thin SEI layer and stable nanostructure after cycling, while the SNC area is gradually damaged and covered with a thick SEI layer after 100 cycles. Our results clearly indicate the improved electrochemical performance and interfacial stability attributed to the vertical graphene coating, and the as-proposed patterning technology also paves a new way for comparative research on coating materials for lithium-ion batteries.

Determination of the Solid Electrolyte Interphase Structure Grown on a Silicon Electrode Using a Fluoroethylene Carbonate Additive

In this work we explore how an electrolyte additive (fluorinated ethylene carbonate – FEC) mediates the thickness and composition of the solid electrolyte interphase formed over a silicon anode in situ as a function of state-of-charge and cycle. We show the FEC condenses on the surface at open circuit voltage then is reduced to C-O containing polymeric species around 0.9 V (vs. Li/Li⁺). The resulting film is about 50 Å thick. Upon lithiation the SEI thickens to 70 Å and becomes more organic-like. With delithiation the SEI thins by 13 Å and becomes more inorganic in nature, consistent with the formation of LiF. This thickening/thinning is reversible with cycling and shows the SEI is a dynamic structure. We compare the SEI chemistry and thickness to 280 Å thick SEI layers produced without FEC and provide a mechanism for SEI formation using FEC additives.

Understanding Surface and Interfacial Chemistry in Functional Nanomaterials via Solid-State NMR

Surface and interfacial chemistry is of fundamental importance in functional nanomaterials applied in catalysis, energy storage and conversion, medicine, and other nanotechnologies. It has been a perpetual challenge for the scientific community to get an accurate and comprehensive picture of the structures, dynamics, and interactions at interfaces. Here, some recent examples in the major disciplines of nanomaterials are selected (e.g., nanoporous materials, battery materials, nanocrystals and quantum dots, supramolecular assemblies, drug-delivery systems, ionomers, and graphite oxides) and it is shown how interfacial chemistry can be addressed through the perspective of solid-state NMR characterization techniques.

Low-Cost and Novel Si-Based Gel for Li-Ion Batteries

Si-based nanostructure composites have been intensively investigated as anode materials for next-generation lithium-ion batteries because of their ultra-high-energy storage capacity. However, it is still a great challenge to fabricate a perfect structure satisfying all the requirements of good electrical conductivity, robust matrix for buffering large volume expansion, and intact linkage of Si particles upon long-term cycling. Here, we report a novel design of Si-based multicomponent three-dimensional (3D) networks in which the Si core is capped with phytic acid shell layers through a facile high-energy ball-milling method. By mixing the functional Si with graphene oxide and functionalized carbon nanotube, we successfully obtained a homogeneous and conductive rigid silicon-based gel through complexation. Interestingly, this Si-based gel with a fancy 3D cross-linking structure could be writable and printable. In particular, this Si-based gel composite delivers a moderate specific capacity of 2711 mA h g^-1 at a current density of 420 mA g^-1 and retained a competitive discharge capacity of more than 800.00 mA h g^-1 at the current density of 420 mA g^-1 after 700 cycles. We provide a new method to fabricate durable Si-based anode material for next-generation high-performance lithium-ion batteries.

Surface-Sensitive NMR Detection of the Solid Electrolyte Interphase Layer on Reduced Graphene Oxide

Forming a stable solid electrolyte interphase (SEI) is critical for rechargeable batteries' performance and lifetime. Understanding its formation requires analytical techniques that provide molecular-level insight. Here, dynamic nuclear polarization (DNP) is utilized for the first time to enhance the sensitivity of solid-state NMR (ssNMR) spectroscopy to the SEI. The approach is demonstrated on reduced graphene oxide (rGO) cycled in Li-ion cells in natural abundance and ¹³C-enriched electrolyte solvents. Our results indicate that DNP enhances the signal of outer SEI layers, enabling detection of natural abundance ¹³C spectra from this component of the SEI on reasonable time frames. Furthermore, ¹³C-enriched electrolyte measurements at 100 K provide ample sensitivity without DNP due to the vast amount of SEI filling the rGO pores, thereby allowing differentiation of the inner and outer SEI layer composition. Developing this approach further will benefit the study of many electrode materials, equipping ssNMR with the necessary sensitivity to probe the SEI efficiently.

Rationalising Heteronuclear Decoupling in Refocussing Applications of Solid-State NMR Spectroscopy

Factors affecting the performance of 1 H heteronuclear decoupling sequences for magic-angle spinning (MAS) NMR spectroscopy of organic solids are explored, as observed by time constants for the decay of nuclear magnetisation under a spin-echo (T2' ). By using a common protocol over a wide range of experimental conditions, including very high magnetic fields and very high radio-frequency (RF) nutation rates, decoupling performance is observed to degrade consistently with increasing magnetic field. Inhomogeneity of the RF field is found to have a significant impact on T2' values, with differences of about 20 % observed between probes with different coil geometries. Increasing RF nutation rates dramatically improve robustness with respect to RF offset, but the performance of phase-modulated sequences degrades at the very high nutation rates achievable in microcoils as a result of RF transients. The insights gained provide better understanding of the factors limiting decoupling performance under different conditions, and the high values of T2' observed (which generally exceed previous literature values) provide reference points for experiments involving spin magnetisation refocussing, such as 2D correlation spectra and measuring small spin couplings.

A Well-Defined Silicon Nanocone-Carbon Structure for Demonstrating Exclusive Influences of Carbon Coating on Silicon Anode of Lithium-Ion Batteries

Nanotechnology and carbon coating have been applied to silicon anodes to achieve excellent lithium-ion batteries, but the exclusive influence of carbon coating on solid-electrolyte interphase (SEI) formation is difficult to exhibit distinctly because of the impurity and morphological irregularity of most nanostructured anodes. Here, we design a silicon nanocone-carbon (SNC-C) composite structure as a model anode to demonstrate the significant influences of carbon coating on SEI formation and electrochemical performance, unaffectedly as a result of pure electrode component and distinctly due to regular nanocone morphology. As demonstrated by morphological and elemental analysis, compared to the SNC electrode, the SNC-C electrode maintains a thinner SEI layer (∼10 nm) and more stable structure during cycling as well as longer cycle life (>725 cycles), higher Coulombic efficiency (>99%), and lower electrode polarization. This well-defined structure clearly shows the interface stability attributed to carbon coating and is promising in fundamental research of the silicon anode.

A bio-inspired nanofibrous silicon/carbon composite as an anode material for lithium-ion batteries

A nanofibrous silicon/carbon composite derived from a cellulose substance was fabricated, showing enhanced electrochemical performances as an anode material for lithium-ion batteries.