Constructing Pure Si Anodes for Advanced Lithium Batteries

A gradient-distributed binder with high energy dissipation for stable silicon anode

Silicon is considered as a promising alternative to traditional graphite anode for lithium-ion batteries. Due to the dramatic volume expansion of silicon anode generated from the insertion of Li+ ions, the binder which can suppress the severe volume change and repeated massive stress impact during cycling is required greatly. Herein, we design a gradient-distributed two-component binder (GE-PAA) to achieve excellent cyclic stability, and reveal the mechanism of high energy dissipative binder stabilized silicon electrodes. The inner layer of the electrode is the polyacrylic acid polymer (PAA) with high Young's modulus, which is used as the skeleton binder to stabilize the silicon particle interface and the electrode structure. The outer layer is the gel electrolyte polymer (GE) with lower Young's modulus, which releases the stress generated during the lithiation and de-lithiation process effectively, achieving the high structural stability at the molecular level and silicon particles. Due to the synergistic effect of the gradient binder design, the silicon electrode retains a reversible capacity of 1557.4 mAh g-1 after 200 cycles at the current density of 0.5 C and 1539.2 mAh g-1 at a high rate of 1.8 C. This work provides a novel binder design strategy for Si anode with long cycle stability.

Achievements, challenges, and perspectives in the design of polymer binders for advanced lithium-ion batteries

Energy storage devices with high power and energy density are in demand owing to the rapidly growing population, and lithium-ion batteries (LIBs) are promising rechargeable energy storage devices. However, there are many issues associated with the development of electrode materials with a high theoretical capacity, which need to be addressed before their commercialization. Extensive research has focused on the modification and structural design of electrode materials, which are usually expensive and sophisticated. Besides, polymer binders are pivotal components for maintaining the structural integrity and stability of electrodes in LIBs. Polyvinylidene difluoride (PVDF) is a commercial binder with superior electrochemical stability, but its poor adhesion, insufficient mechanical properties, and low electronic and ionic conductivity hinder its wide application as a high-capacity electrode material. In this review, we highlight the recent progress in developing different polymeric materials (based on natural polymers and synthetic non-conductive and electronically conductive polymers) as binders for the anodes and cathodes in LIBs. The influence of the mechanical, adhesion, and self-healing properties as well as electronic and ionic conductivity of polymers on the capacity, capacity retention, rate performance and cycling life of batteries is discussed. Firstly, we analyze the failure mechanisms of binders based on the operation principle of lithium-ion batteries, introducing two models of "interface failure" and "degradation failure". More importantly, we propose several binder parameters applicable to most lithium-ion batteries and systematically consider and summarize the relationships between the chemical structure and properties of the binder at the molecular level. Subsequently, we select silicon and sulfur active electrode materials as examples to discuss the design principles of the binder from a molecular structure point of view. Finally, we present our perspectives on the development directions of binders for next-generation high-energy-density lithium-ion batteries. We hope that this review will guide researchers in the further design of novel efficient binders for lithium-ion batteries at the molecular level, especially for high energy density electrode materials.

Facile Lithium Densification Kinetics by Hyperporous/Hybrid Conductor for High-Energy-Density Lithium Metal Batteries

Lithium metal anode (LMA) emerges as a promising candidate for lithium (Li)-based battery chemistries with high-energy-density. However, inhomogeneous charge distribution from the unbalanced ion/electron transport causes dendritic Li deposition, leading to "dead Li" and parasitic reactions, particularly at high Li utilization ratios (low negative/positive ratios in full cells). Herein, an innovative LMA structural model deploying a hyperporous/hybrid conductive architecture is proposed on single-walled carbon nanotube film (HCA/C), fabricated through a nonsolvent induced phase separation process. This design integrates ionic polymers with conductive carbon, offering a substantial improvement over traditional metal current collectors by reducing the weight of LMA and enabling high-energy-density batteries. The HCA/C promotes uniform lithium deposition even under rapid charging (up to 5 mA cm-2) owing to its efficient mixed ion/electron conduction pathways. Thus, the HCA/C demonstrates stable cycling for 200 cycles with a low negative/positive ratio of 1.0 when paired with a LiNi0.8Co0.1Mn0.1O2 cathode (areal capacity of 5.0 mAh cm-2). Furthermore, a stacked pouch-type full cell using HCA/C realizes a high energy density of 344 Wh kg-1 cell/951 Wh L-1 cell based on the total mass of the cell, exceeding previously reported pouch-type full cells. This work paves the way for LMA development in high-energy-density Li metal batteries.

Bio-Inspired Electrodes with Rational Spatiotemporal Management for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are currently the predominant energy storage power source. However, the urgent issues of enhancing electrochemical performance, prolonging lifetime, preventing thermal runaway-caused fires, and intelligent application are obstacles to their applications. Herein, bio-inspired electrodes owning spatiotemporal management of self-healing, fast ion transport, fire-extinguishing, thermoresponsive switching, recycling, and flexibility are overviewed comprehensively, showing great promising potentials in practical application due to the significantly enhanced durability and thermal safety of LIBs. Taking advantage of the self-healing core-shell structures, binders, capsules, or liquid metal alloys, these electrodes can maintain the mechanical integrity during the lithiation-delithiation cycling. After the incorporation of fire-extinguishing binders, current collectors, or capsules, flame retardants can be released spatiotemporally during thermal runaway to ensure safety. Thermoresponsive switching electrodes are also constructed though adding thermally responsive components, which can rapidly switch LIB off under abnormal conditions and resume their functions quickly when normal operating conditions return. Finally, the challenges of bio-inspired electrode designs are presented to optimize the spatiotemporal management of LIBs. It is anticipated that the proposed electrodes with spatiotemporal management will not only promote industrial application, but also strengthen the fundamental research of bionics in energy storage.

Engineering a Low-Strain Si@TiSi2@NC Composite for High-Performance Lithium-Ion Batteries

The huge volume expansion/contraction of silicon (Si) during the lithium (Li) insertion/extraction process, which can lead to cracking and pulverization, poses a substantial impediment to its practical implementation in lithium-ion batteries (LIBs). The development of low-strain Si-based composite materials is imperative to address the challenges associated with Si anodes. In this study, we have engineered a TiSi2 interface on the surface of Si particles via a high-temperature calcination process, followed by the introduction of an outermost carbon (C) shell, leading to the construction of a low-strain and highly stable Si@TiSi2@NC composite. The robust TiSi2 interface not only enhances electrical and ionic transport but also, more critically, significantly mitigates particle cracking by restraining the stress/strain induced by volumetric variations, thus alleviating pulverization during the lithiation/delithiation process. As a result, the as-fabricated Si@TiSi2@NC electrode exhibits a high initial reversible capacity (2172.7 mAh g-1 at 0.2 A g-1), superior rate performance (1198.4 mAh g-1 at 2.0 A g-1), and excellent long-term cycling stability (847.0 mAh g-1 after 1000 cycles at 2.0 A g-1). Upon pairing with LiNi0.6Co0.2Mn0.2O2 (NCM622), the assembled Si@TiSi2@NC||NCM622 pouch-type full cell exhibits exceptional cycling stability, retaining 90.1% of its capacity after 160 cycles at 0.5 C.

Well-Dispersed Bi nanoparticles for promoting the lithium storage performance of Si Anode: Effect of the bridging Bi nanoparticles

Silicon (Si) is considered a promising anode material for lithium-ion batteries (LIBs) due to its high theoretical specific capacity of up to 4200 mAh/g. However, the poor cycling and rate performances of Si induced by the low intrinsic electronic conductivity and large volume expansion during the lithiation/delithiation process limit its practical application. Herein, a novel silicon/bismuth@nitrogen-doped carbon (Si/Bi@NC) composite with nanovoids was synthesized and investigated as an advanced anode material for LIBs. In such a structure, ultrafine bismuth nanoparticles coupled with an N-doped carbon layer were introduced to modify the surface of Si nanoparticles. Subsequently, the lithiated LixBi has excellent high ionic conductivity and acts as a fast transport bridge for lithium ions. The introduced carbon coating layer and nanovoids can buffer the volume expansion of Si during the lithiation/delithiation process, thus maintaining structural stability during the cycling process. As a result, the Si/Bi@NC composite exhibits excellent electrochemical performance, providing a relatively high capacity of 955.8 mAh/g at 0.5 A/g after 450 cycles and excellent rate performance with a high capacity of 477.8 mAh/g even at 10.0 A/g. Furthermore, the assembled full cell with LiFePO4 as cathode and pre-lithium Si/Bi@NC as anode can provide a high capacity of 138.8 mAh/g at 1C after 90 cycles, exhibiting outstanding cycling performance.

Formulating Electron Beam-Induced Covalent Linkages for Stable and High-Energy-Density Silicon Microparticle Anode

High-capacity silicon (Si) materials hold a position at the forefront of advanced lithium-ion batteries. The inherent potential offers considerable advantages for substantially increasing the energy density in batteries, capable of maximizing the benefit by changing the paradigm from nano- to micron-sized Si particles. Nevertheless, intrinsic structural instability remains a significant barrier to its practical application, especially for larger Si particles. Here, a covalently interconnected system is reported employing Si microparticles (5 µm) and a highly elastic gel polymer electrolyte (GPE) through electron beam irradiation. The integrated system mitigates the substantial volumetric expansion of pure Si, enhancing overall stability, while accelerating charge carrier kinetics due to the high ionic conductivity. Through the cost-effective but practical approach of electron beam technology, the resulting 500 mAh-pouch cell showed exceptional stability and high gravimetric/volumetric energy densities of 413 Wh kg-1, 1022 Wh L-1, highlighting the feasibility even in current battery production lines.