Lithium-ion Batteries Enabled by Silicon Anodes
2: Battery Science Branch, Energy and Biotechnology, Division Sensor and Electron Devices Directorate, USA Army Research Lab., Adelphi, MD, USA
Deploying lithium-ion (Li-ion) batteries depends on cost-effective electrode materials with high energy and power density to facilitate lower weight and volume. Si-based anode materials theoretically offer superior lithium storage capacity. Replacing a graphite anode with high-capacity materials such as silicon will further improve the energy density. Durable, low-cost, and high-energy-density materials are vital to developing plug-in electric vehicles as affordable and convenient as gasoline-powered ones, while reducing carbon emissions. This reference presents the knowledge gained over recent decades in the materials science and chemistry of silicon and its derivates as anode materials for Li-ion batteries, and provides insights into developing Si-based anode materials for next-generation batteries. Coverage includes the structure and chemistry of silicon, electrolytes and chemistry of Si anodes, nanostructure and binder additives for Si anodes, surface modification and mechanical properties. Researchers in academia and industry will find this detailed reference a highly useful resource.
Inspec keywords: lithium compounds; electrochemical electrodes; elemental semiconductors; silicon compounds; silicon
Other keywords: Si; elemental semiconductors; secondary cells; impact ionisation; anodes; anodisation; silicon compounds; electrochemical electrodes; boron; silicon; semiconductor doping
Subjects: Elemental semiconductors; Textbooks; Secondary cells; General electrical engineering topics; Secondary cells; Handbooks and dictionaries; Electrochemistry and electrophoresis; Education and training; Monographs, and collections
- Book DOI: 10.1049/PBPO156E
- Chapter DOI: 10.1049/PBPO156E
- ISBN: 9781785619557
- e-ISBN: 9781785619564
- Page count: 472
- Format: PDF
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Front Matter
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1 Overview of the development of silicon anodes for lithium-ion batteries
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Si-based anode materials are currently by far the most studied negative electrode (anode) materials for the purpose of increasing Li-ion battery energy density. Si has nearly three times the volumetric capacity as graphite, the anode material conventionally used in Li-ion batteries today. Such an increase in anode capacity could theoretically increase the energy density of Li-ion batteries by as much as 34 percent. Si-based anodes are also potentially less expensive than graphite: in 2017 the cost of Si in the US was about 2.6 USD/kg, whereas the cost of battery. The possibility of enabling higher energy density with similar costs or even at a cost reduction compared to cells with conventional anodes is a strong driver for research and development work in this field.
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2 Application of Zintl–Klemm rules to silicon-based LIB anodes
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Silicon is an earth abundant element that has been at the core of numerous scientific advances ranging from microelectronics to photovoltaics to energy storage. In the area of energy storage, it has been studied due to its combination of high theoretical capacity and low voltage that, when paired with high-capacity cathodes, is capable of generating a high-energy-density system capable of meeting next-generation energy-storage needs. With this great promise comes important limitations that have yet to allow widespread introduction of silicon into lithium-ion cells. These issues include the large (~350 percent) volume expansion that occurs going from elemental silicon to Li15Si4 (the room temperature endpoint in an electrochemical lithiation), electrolyte reactivity, solid electrolyte interphase (SEI) layer stability, the role of silicon purity, and parasitic side reactions that irreversibly remove active lithium from the cathode. This chapter addresses recent progress in under-standing the reactivity of the silicon at various states of charge and how this reactivity manifests itself in an electrochemical cell and in the cycling stability of silicon and lithiated silicides.
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3 Electrochemistry of silicon
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Learning from the seminal works, we have a clear picture of the phase evolutions and localized atomic structural changes associated with the Si lithiation/delithiation process. This knowledge can guide us on the right path to the successful utilization of Si anodes. For instance, it is suggested to limit the cutoff potential above the crystallization of a-LixSi to avoid the two-phase delithiation of c-Li15Si4 process, which is more challenging than the delithiation of a-LixSi. The existence of surface oxide and its reduction products LixSiOy should also be considered when designing binder or solid electrolyte interface SEI, because mostly those determine interfacial bonding. It is believed that stable cycling of Si-based anodes with high coulombic efficiencies will be realized in the near future with dedicated effort in electrode and/or electrolyte design.
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4 Electrolytes used in silicon anodes
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As the most versatile and attractive energy storage system, lithium-ion batteries (LIBs) continue to draw vast attention because of their high energy density, low self-discharge property, nearly zero- memory effect, high open-circuit voltage, and long lifespan. While LIBs promoted worldwide development of the portable electronic devices that have profoundly changed our daily lives and remain the most viable choice for electric vehicles (EVs) and hybrid EVs, their limited capacities (no higher than 200 mAh/g) hinder wider transportation applications. Silicon (Si) emerges as one of the most promising next-generation anode materials by providing impressive gravimetric (3579 mAh/g, based on Li15Si4) and volumetric (9786 mAh/cm3, based on the initial volume of Si) capacities, as well as high abundance, low cost, negligible toxicity, high safety, and low operating potential (0.2-0.4 V vs. Li/Li+). However, such extraordinarily high capacity comes with a catch, that is Si- anodes suffer up to a 400 percent volume expansion upon lithiation, which results in particle pulverization, loss of conductivity, and an unstable electrode-electrolyte interface, essentially leading to fast capacity fading. While approaches have been actively engaged to mitigate this issue, including engineering Si particles and surrounding components, significant efforts are still needed to tackle this important barrier.
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5 Interfacial chemistry on silicon anode
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This chapter is mainly focused on the surface chemistry and structure of the silicon (Si) anode. Because of its ultrahigh theoretical lithium (Li) storage capacity is considered one of the most promising alternatives to graphite anodes for Li-ion batteries. Unlike graphite anode, Si anode does intercalate Li cations upon lithiation. Instead, Si alloys with Li, resulting in a drastic structural disruption (e.g., amorphization of crystal Si (c-Si) as a starting material). This disruption leads to up to a 300 percent volumetric change and structural morpho-logical evolution, resulting in an intrinsically unstable solid-electrolyte interphase (SEI) on the Si anode surface, further capacity fading, and limited cycle life.
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6 Computational studies for understanding and developing silicon anodes
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Energy storage technology is key for a rapid transition into a global automotive market dominated by electric vehicles (EVs). Therefore, it is necessary to develop revolutionary next-generation lithium-ion batteries (LiBs) that can sustain an increase in driving range for EVs. Graphite-based LiBs are quickly reaching their physical limit, but silicon (Si) is an ideal replacement due to its high Li storage capacity. Indeed, Si is one of the most promising anode materials in LiBs mainly due to their abundance and high theoretical gravimetric specific capacity (4200 mAh/g). Despite this increased capacity over carbonaceous materials, one of silicon's main roadblocks for its implementation as an anode material is the huge volume expansion and contraction that takes place upon cycling. Moreover, the work function of the Si anodes is approximately 0.37 V vs. Li+/Li, and at these potentials most of the typical electrolyte solutions mainly composed of a salt in solution of carbonate solvents become easily reduced at the surface of the anode. Therefore, during the normal operation of a battery, a multicomponent layer known as the solid-electrolyte interphase (SEI) layer forms as a result of the breaking down of molecular bonds of salts and solvent molecules making up the electrolyte solution. Ideally, this SEI layer should act like a sponge, blocking electrons and molecules, allowing passage to ions, and protecting the battery from degradation over time. Thus, designing this layer in a specific way is a promising solution to improve battery life.
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7 Nanostructure silicon for Li-ion batteries
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Rechargeable lithium-ion batteries (LIBs) have been widely used for portable electronics and electric vehicles, and they have shown great promise for large-scale stationary energy storage. The advancement of next-generation LIBs with lower cost and higher energy density greatly depends on the development of low-cost, energy-dense electrode materials. Intense research has been devoted to developing higher-capacity cathode and anode materials. Sulfur and oxygen-based materials have been intensely explored for the cathode while silicon (Si), tin, and Li are mainly studied for the anode side. Among these materials, Si, as the second most abundant element in the earth, is cheap, nontoxic and most importantly possesses a theoretical capacity that is an order magnitude higher than the conventional graphite anode. Successful implementation of a Si anode will fundamentally enable high-energy Li battery and reduce the cell cost. Despite its high capacity, Si suffers from short cycling life, which is caused by its large volume change during lithiation/ delithiation processes and the serious issues stemming from this volume change, such as unstable solid electrolyte interphase (SEI), particle fracture, and cracking of the electrode structure. During the past decade, nanotechnology has been providing unparallel solutions to solve these challenges. First, by reducing the size of Si to the nanoscale in at least one dimension, the stress induced upon lithiation/delithiation can be effectively alleviated, which can even prevent particle fracture. In addition, nanostructures offer a facilitated ion/electron transport and thus enhance the rate capability. Following the initial insight of Si nanowires (NWs), various Si nanostructures including nanoparticles (NPs), nanorods (NRs), NWs, nanotubes (NTs), and nanosheets (NSs) have been developed and demonstrated an improved cycling life compared to their bulk counterparts.
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8 Binder additive for silicon anodes
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Polymer electrode binders are used to adhere battery active material (AM) powders and conductive additive powders to each other in order to form a composite electrode, as well as to adhere this composite to the current collector. This provides mechanical stability to the laminate as well as permanent electrical connections in the composite. Many polymer electrode binders have been discussed in literature, but only a few types have found commercial success in lithium-ion battery applications. This chapter will cover both polymer binders for commercial lithium-ion batteries as well as new types of binders that have been proposed in literature for use in Si electrodes.
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9 Surface modification for silicon anodes
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Surface coating has previously been applied to Si alloy anode materials, to mitigate unfavourable reduction of electrolyte. Among coating materials, carbon has been widely used to promote stable SEI formation and enhance electronic conductivity. On the other hand, coating Si particles with metal oxides, such as Al2O3 and TiO2, has shown improved cycling performance, especially in the suppression of side reactions at Si anode surface. This chapter will discuss the details of the functionalities of the selected coating materials, as well as the coating techniques for scalable surface chemistry modification.
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10 Mechanical characterization of silicon-based electrodes
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The substantial volume change of silicon (Si) during repeated lithiation/delithiation cycling can cause large mechanical stresses that fracture the Si-based electrodes, resulting in electrical isolation of Si particles and consuming Li and electrolytes by continuously forming solid electrolyte interphase (SEI). These factors lead to the fast capacity fading of Si-based electrodes. To mitigate the electromechanical degradation of Si electrodes, it is necessary to understand the mechanical property evolution of Si electrodes during cycling.
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11 Practical implementation of silicon-based negative electrodes in lithium-ion full-cells—challenges and solutions
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In this chapter, we will provide the fundamental insights for the practical implementation of Si-based negative electrode materials in LIB full-cells, address the major challenges and give guidance for future approaches to achieve the targets in terms of the battery's key performance metrics in commercial cell formats.
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12 A silicon future
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The birth of lithium-ion battery (LIB) in early 1990 marked a revolutionary milestone in the history of batteries. It not only set a record in energy density among all rechargeable batteries known at the time, more importantly, it was the first battery based entirely on intercalation chemistries. It was this latter feature that ensures the unprecedented reversibility of the cell reactions, as reflected by the cycle-life in the order of over thousands of times.
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Back Matter
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