Nanoscale Electrical Degradation of Silicon-Carbon Composite Anode Materials for Lithium-Ion Batteries

Characterisation of defects in porous silicon as an anode material using positron annihilation Doppler Broadening Spectroscopy

Here we present Positron Annihilation Doppler Broadening Spectroscopy (PADBS) as a powerful method to analyse the origin and development of defect processes in porous silicon structures as a result of alloying with lithium for the use in battery anode applications. Several prepared anodes were lithiated (discharged against Li+/Li) and de-lithiated (charged) with different capacities followed by a distinct treatment procedure and an analysis using the Delft Variable Energy Positron Beam. The results presented here show that we can distinguish two different processes attributed to (1) structural changes in silicon as a result of the alloying process, and (2) the formation of defects that initiate degradation of the material. The limit at which the porous material can be used for at least the first two cycles without the occurrence of damage can thus be accurately determined by using the PADBS technique.

Nanoscale Visualization of the Electron Conduction Channel in the SiO/Graphite Composite Anode

Conductive atomic force microscopy (C-AFM) is widely used to determine the electronic conductivity of a sample surface with nanoscale spatial resolution. However, the origin of possible artifacts has not been widely researched, hindering the accurate and reliable interpretation of C-AFM imaging results. Herein, artifact-free C-AFM is used to observe the electron conduction channels in Si-based composite anodes. The origin of a typical C-AFM artifact induced by surface morphology is investigated using a relevant statistical method that enables visualization of the contribution of artifacts in each C-AFM image. The artifact is suppressed by polishing the sample surface using a cooling cross-section polisher, which is confirmed by Pearson correlation analysis. The artifact-free C-AFM image was used to compare the current signals (before and after cycling) from two different composite anodes comprising single-walled carbon nanotubes (SWCNTs) and carbon black as conductive additives. The relationship between the electrical degradation and morphological evolution of the active materials depending on the conductive additive is discussed to explain the improved electrical and electrochemical properties of the electrode containing SWCNTs.

Root Reason for the Failure of a Practical Zn-Ni Battery: Shape Changing Caused by Uneven Current Distribution and Zn Dissolution

In recent years, with the increasing application of lithium-ion batteries in energy storage devices, fire accidents caused by lithium-ion batteries have become more frequent and have arisen wide concern. Due to the safety of aqueous electrolyte, aqueous Zn-based batteries have attracted vast attention, among which Zn-Ni batteries stand out by virtue of their excellent rate performance and environmental friendliness. However, poor cycling life limits the application of Zn-Ni batteries. To figure out the main cause, a failure analysis of a practical Zn-Ni battery has been carried out. During the cycling of the Zn-Ni battery, the evolution of gas, the shape changing, and the aggregation of additive and binder of Zn anode can be observed. Combined with the finite element analysis, we finally reveal that the key factor of battery failure is the shape changing of the Zn anode caused by uneven current distribution and the dissolution of Zn. The shape changing of the Zn anode reduces the effective surface area of anode and increases the possibility of dead Zn, which makes the battery unable to discharge even in the presence of a large amount of Zn. These findings are helpful to deepen the understanding of the working and failure mechanisms of the Zn anode and provide effective guidance for subsequent research.

Three-Dimensional Mapping of Resistivity and Microstructure of Composite Electrodes for Lithium-Ion Batteries

Nanoparticle silicon-graphite composite electrodes are a viable way to advance the cycle life and energy density of lithium-ion batteries. However, characterization of composite electrode architectures is complicated by the heterogeneous mixture of electrode components and nanoscale diameter of particles, which falls beneath the lateral and depth resolution of most laboratory-based instruments. In this work, we report an original laboratory-based scanning probe microscopy approach to investigate composite electrode microstructures with nanometer-scale resolution via contrast in the electronic properties of electrode components. Applying this technique to silicon-based composite anodes demonstrates that graphite, SiO_x nanoparticles, carbon black, and LiPAA binder are all readily distinguished by their intrinsic electronic properties, with measured electronic resistivity closely matching their known material properties. Resolution is demonstrated by identification of individual nanoparticles as small as ∼20 nm. This technique presents future utility in multiscale characterization to better understand particle dispersion, localized lithiation, and degradation processes in composite electrodes for lithium-ion batteries.

Understanding Protection Mechanisms of Graphene-Encapsulated Silicon Anodes with Operando Raman Spectroscopy

Carbon-coated silicon micro- and nanostructures have been widely used as composite anodes for lithium-ion batteries combining the benefits of high theoretical capacity of Si and better conductivity of carbon. To optimize structures that allow the Si volume expansion without losing the electrical connection, a detailed carbon protection mechanism is desired. We fabricate a network of interconnected sandwich branches with a silicon thin film encapsulated between a porous 3-dimensional graphene foam and graphene drapes (so-called a graphene ensemble). This prototype binder-free anode, of great mechanical strength and composed of only silicon and few-layer graphene, provides distinct signals under operando Raman spectroscopy. During electrochemical cycles, the graphene G peak shows variation of peak position and intensity, while the 2D peak experiences a negligible shift from limited deformation. Silicon displays excellent structural reversibility under the sandwich protection, validating the functions of graphenic carbon coating. This specific graphene ensemble can also serve as an experimental scaffold for mechanical and chemical analysis of many active materials.

Integration of Graphite and Silicon Anodes for the Commercialization of High-Energy Lithium-Ion Batteries

Silicon is considered a most promising anode material for overcoming the theoretical capacity limit of carbonaceous anodes. The use of nanomethods has led to significant progress being made with Si anodes to address the severe volume change during (de)lithiation. However, less progress has been made in the practical application of Si anodes in commercial lithium-ion batteries (LIBs). The drastic increase in the energy demands of diverse industries has led to the co-utilization of Si and graphite resurfacing as a commercially viable method for realizing high energy. Herein, we highlight the necessity for the co-utilization of graphite and Si for commercialization and discuss the development of graphite/Si anodes. Representative Si anodes used in graphite-blended electrodes are covered and a variety of strategies for building graphite/Si composites are organized according to their synthetic methods. The criteria for the co-utilization of graphite and Si are systematically presented. Finally, we provide suggestions for the commercialization of graphite/Si combinations.

Scanning Spreading Resistance Microscopy: A Promising Tool for Probing the Reaction Interface of Li-Ion Battery Materials

Imaging of the Li-insertion/extraction [Li-in/out] interface of the electrode materials of Li-ion batteries is essential to reveal their bulk mechanism of electrochemical reaction and phase behavior in the crystal. Generally, the material properties significantly change at this interface. Therefore, direct probing of the changing properties is a promising approach to reliably investigate the Li-in/out interface in the bulk crystal of electrode materials. In this study, we investigated the change in electron conductivity of rutile-TiO₂ with Li-insertion and extraction, as a model for the electrochemical interface of a bulk crystal of electrode material. In addition, we probed the interface using logarithm contact resistance [log  R (Ω)] imaging via scanning spreading resistance microscopy (SSRM). A distinct Li-in/out interface on the rutile-TiO₂(001) wafer was observed using this technique. The imaging resolution of this region was estimated to be approximately 40-50 nm in SSRM images, which was two to three times higher than the resolution of the topographic image (100-150 nm), which was restricted to the curvature radius of the SSRM probe tip. A high spatial resolution was obtained via SSRM imaging because this approach is not influenced by the geometric effects of the surface. This result demonstrated the potential of SSRM imaging for the study of the Li-in/out interface.

A Facile Strategy to Construct Silver-Modified, ZnO-Incorporated and Carbon-Coated Silicon/Porous-Carbon Nanofibers with Enhanced Lithium Storage

For Si anode materials used for lithium ion batteries (LIBs), developing an effective solution to overcome their drawbacks of large volume change and poor electronic conductivity is highly desirable. Here, the composites of ZnO-incorporated and carbon-coated silicon/porous-carbon nanofibers (ZnO-Si@C-PCNFs) are designed and synthesized via a traditional electrospinning method. The prepared ZnO-Si@C-PCNFs can obviously overcome these two drawbacks and provide excellent LIB performance with excellent rate capability and stable long cycling life of 1000 cycles with reversible capacity of 1050 mA h g^-1 at 800 mA g^-1 . Meanwhile, anodes of ZnO-Si@C-PCNFs attached with Ag particles display enhanced LIB performance, maintaining an average capacity of 920 mA h g^-1 at a large current of 1800 mA g^-1 even for 1000 cycles with negligible capacity loss and excellent reversibility. In addition, the assembling method with important practical significance for a simple pouch full cell is designed and used to evaluate the active materials. The Ag/ZnO-Si@C-PCNFs are prelithiated and assembled in full cells using LiNi_0.5 Co_0.2 Mn_0.3 O₂ (NCM523) as cathodes, exhibiting higher energy density (230 W h kg^-1 ) of 18% than that of 195 W h kg^-1 for commercial graphite//NCM523 full pouch cells. Importantly, the comprehensive mechanisms of enhanced electrochemical kinetics originating from ZnO-incorporation and Ag-attachment are revealed in detail.