Nanomaterials for Rechargeable Lithium Batteries** Peter G
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Reviews P. G. Bruce et al. DOI: 10.1002/anie.200702505 Lithium Batteries Nanomaterials for Rechargeable Lithium Batteries** Peter G. Bruce,* Bruno Scrosati, and Jean-Marie Tarascon Keywords: electrochemistry · lithium · nanoelectrodes · nanomaterials · polymers Angewandte Chemie 2930 www.angewandte.org 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008, 47, 2930 – 2946 Angewandte Lithium Batteries Chemie Energy storage is more important today than at any time in human From the Contents history. Future generations of rechargeable lithium batteries are required to power portable electronic devices (cellphones, laptop 1. Introduction 2931 computers etc.), store electricity from renewable sources, and as a vital 2. Advantages and Disadvantages component in new hybrid electric vehicles. To achieve the increase in of Nanomaterials for Lithium energy and power density essential to meet the future challenges of Batteries 2932 energy storage, new materials chemistry, and especially new nano- materials chemistry, is essential. We must find ways of synthesizing 3. Negative Electrodes 2932 new nanomaterials with new properties or combinations of properties, 4. Electrolytes 2938 for use as electrodes and electrolytes in lithium batteries. Herein we review some of the recent scientific advances in nanomaterials, and 5. Positive Electrodes 2940 especially in nanostructured materials, for rechargeable lithium-ion 6. Three-Dimensional Batteries batteries. with Nanostructured Electrodes 2944 7. Supercapacitors and Fuel Cells 2944 1. Introduction 8. Summary and Outlook 2945 The storage of electrical energy will be far more important in this century than it was in the last. Whether to power the myriad portable consumer electronic devices (cell phones, lated from the layered LiCoO2 intercalation host, pass across PDAs, laptops, or for implantable medical applications, such the electrolyte, and are intercalated between the graphite as artificial hearts, or to address global warming (hybrid layers in the anode. Discharge reverses this process. The electric vehicles, storage of wind/solar power), the need for electrons, of course, pass around the external circuit. The clean and efficient energy storage will be vast. Nanomaterials rechargeable lithium battery is a supreme representation of have a critical role to play in achieving this change in the way solid-state chemistry in action. A more detailed account of we store energy. lithium-ion batteries than is appropriate here may be Rechargeable lithium batteries have revolutionized port- obtained from the literature.[1–3] able electronic devices. They have become the dominant The first-generation lithium-ion battery has electrodes power source for cell phones, digital cameras, laptops etc., that are composed of powders containing millimeter-sized because of their superior energy density (capability to store 2– particles, and the electrolyte is trapped within the millimeter- 3 times the energy per unit weight and volume compared with sized pores of a polypropylene separator. Although the conventional rechargeable batteries). The worldwide market battery has a high energy density, it is a low-power device for rechargeable lithium batteries is now valued at 10 billion (slow charge/discharge). No matter how creative we are in dollars per annum and growing. They are the technology of designing new lithium intercalation hosts with higher rates, choice for future hybrid electric vehicles, which are central to limits exist because of the intrinsic diffusivity of the lithium À8 2 À1 the reduction of CO2 emissions arising from transportation. ion in the solid state (ca. 10 cm s ), which inevitably limits The rechargeable lithium battery does not contain lithium the rate of intercalation/deintercalation, and hence charge/ metal. It is a lithium-ion device, comprising a graphite discharge. However, an increase in the charge/discharge rate negative electrode (anode), a non-aqueous liquid electrolyte, of lithium-ion batteries of more than one order of magnitude and a positive electrode (cathode) formed from layered is required to meet the future demands of hybrid electric LiCoO2 (Figure 1). On charging, lithium ions are deinterca- vehicles and clean energy storage. Nanomaterials, so often [*] Prof. P. G. Bruce School of Chemistry University of St. Andrews St. Andrews, Fife, KY16 9ST (UK) Fax : (+ 44)1334-463808 E-mail: [email protected] Prof. B. Scrosati Dipartimento di Chimica Università di Roma Rome (Italy) Prof. J.-M. Tarascon Laboratoire de Reactivite et de Chimie des Solides Figure 1. Schematic representation of a lithium-ion battery. Negative Universite de Picardie Amiens (France) electrode (graphite), positive electrode (LiCoO2), separated by a non- aqueous liquid electrolyte. [**] Thanks to Dr. Aurelie Debart for preparation of the frontispiece. Angew. Chem. Int. Ed. 2008, 47, 2930 – 2946 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2931 Reviews P. G. Bruce et al. hyped or misrepresented by claims of delivering new proper- cles; for example, reversible lithium intercalation into ties, have the genuine potential to make a significant impact mesoporous b-MnO2 without destruction of the rutile on the performance of lithium-ion batteries, as their reduced structure.[5] dimensions enable far higher intercalation/deintercalation 2. The reduced dimensions increases significantly the rate of rates and hence high power. This is just one property that may lithium insertion/removal, because of the short distances be enhanced by the use of nanomaterials. However, nano- for lithium-ion transport within the particles. The charac- materials are certainly not a panacea. The advantages and teristic time constant for diffusion is given by t = L2/D, disadvantages of lithium-ion battery materials are summar- where L is the diffusion length and D the diffusion ized in Section 2, and thereafter advances in the use of constant. The time t for intercalation decreases with the nanomaterials, emphasizing in particular nanostructured square of the particle size on replacing micrometer with materials, as negative electrodes, electrolytes, and positive nanometer particles.[4] electrodes for rechargeable lithium batteries are described.[4] 3. Electron transport within the particles is also enhanced by The illustrative examples that are presented are mainly from nanometer-sized particles, as described for lithium ions.[4] the work of the authors. 4. A high surface area permits a high contact area with the electrolyte and hence a high lithium-ion flux across the interface. 2. Advantages and Disadvantages of Nanomaterials 5. For very small particles, the chemical potentials for lithium for Lithium Batteries ions and electrons may be modified, resulting in a change of electrode potential (thermodynamics of the reaction).[6] Advantages 6. The range of composition over which solid solutions exist 1. They enable electrode reactions to occur that cannot take is often more extensive for nanoparticles,[7] and the strain place for materials composed of micrometer-sized parti- associated with intercalation is often better accommo- dated. Peter Bruce is Professor of Chemistry at the University of St Andrews,Scotland. His Disadvantages research interests embrace the synthesis and 1. Nanoparticles may be more difficult to synthesize and characterization of materials (extended their dimensions may be difficult to control. arrays and polymers) with new properties or combinations of properties,and in particular 2. High electrolyte/electrode surface area may lead to more materials for new generations of energy significant side reactions with the electrolyte, and more conversion and storage devices. He has difficulty maintaining interparticle contact. received a number of awards and fellow- 3. The density of a nanopowder is generally less than the ships,and is a fellow of the Royal Society. same material formed from micrometer-sized particles. The volume of the electrode increases for the same mass of material thus reducing the volumetric energy density. Bruno Scrosati is Professor of Electrochemis- try at the University of Rome. He has been president of the International Society of Solid State Ionics,the Italian Chemical 3. Negative Electrodes Society,and the Electrochemical Society, and is fellow of the Electrochemical Society 3.1. Nanoparticles (ECS) and of the International Society of Electrochemistry (ISE). He has a “honoris Graphite powder, composed of micrometer-sized parti- causa” (honorary DSc) from the University cles, has been the stalwart of negative electrodes for of St. Andrews in Scotland. He won the XVI rechargeable lithium batteries for many years.[1,2] Replace- Edition of the Italgas Prize,Science and Environment. He is European editor of the ment by nanoparticulate graphite would increase the rate of Journal of Power Sources and member of lithium insertion/removal and thus the rate (power) of the the editorial boards of various international battery. Lithium is inserted into graphite at a potential of less journals. than 1 V versus Li+/Li. At such low potentials, reduction of Jean-Marie Tarascon is Professor at the Uni- the electrolyte occurs, accompanied by the formation of a versity of Picardie (Amiens). He develops passivating (solid electrolyte interface) layer on the graphite techniques for the synthesis of electronic surface.[8–10] The formation of such a layer is essential for the materials (superconductors,ferroelectrics, operation of graphite electrodes, as it inhibits exfoliation. The