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An Overview of Transition Metal Oxides for Electrochemical Energy Storage

Ethan C. Self1, Devendrasinh Darbar1, Veronica Augustyn2, and Jagjit Nanda1

1Oak Ridge National Laboratory, Chemical Sciences Division, Oak Ridge, TN, 37831, USA

2North Carolina State University, Department of Materials Science and Engineering, Raleigh, NC, 27695, USA

Transition metal oxides (TMOs) are used in many commercial and research applications, including catalysis, electrochemical energy storage/conversion, electronics, and thermoelectrics. This book focuses on TMOs for electrochemical energy storage devices with particular emphasis on intercalation-based secondary (rechargeable) batteries. This introductory chapter provides a broad overview of such applications, and detailed treatments of specific subjects are given in Chapters 2-16.

1.1 Fundamentals of Electrochemical Cells

An electrochemical cell consists of two electrodes (denoted as cathode/anode or positive/negative) separated by an ionically conductive, electronically insulating electrolyte. Batteries convert chemical energy into electrical energy through Faradaic charge transfer processes where: (i) oxidation/reduction reactions occur within anode/cathode active materials and (ii) electrons are transported through an external circuit to maintain charge neutrality at each electrode. These reactions are irreversible in primary batteries (e.g. Zn-MnO2 and Li-MnO2) designed for single-use applications. On the other hand, secondary batteries (e.g. lead-acid, nickel-metal hydride, and Li-ion) leverage reversible redox processes and can be repeatedly charged/discharged, a requirement for many end-use applications (e.g. electric vehicles).

Electrochemical capacitors are another form of energy storage devices which provide specific energy and power between that of dielectric capacitors and rechargeable batteries. Supercapacitors store/deliver energy through non-Faradaic processes where ions are stored in the electrochemical double layer near the electrode surfaces. On the other hand, pseudocapacitive materials store energy through charge transfer reactions which may include: (i) oxidation/reduction of the electrode surface and/or (ii) intercalation of ions into a host active material. Hybrid configurations utilizing pseudocapacitive materials approach the specific energy of rechargeable batteries.

Figure 1.1 Ragone plot showing the specific energy and power of several electrochemical energy storage systems.

Source: Image reproduced from Hayner et al. [1], International Energy Agency, Technology Roadmaps: Electric and Plug-in Hybrid Electric Vehicles, 2009, p. 12. (Original source: Johnson Control - SAFT 2005 and 2007.)

Two important performance metrics of energy storage devices are the specific energy (Wh kg-1) and specific power (W kg-1), which describe how much and how quickly energy can be stored/delivered, respectively. Analogous quantities normalized to system volume (i.e. energy/power densities with units of Wh L-1 and W L-1) are also commonly used. Ragone plots (Figure 1.1) summarize these energy/power relationships and are useful to assess the viability of different energy storage platforms for a given application. Figure 1.1 shows a fundamental tradeoff between a system's specific energy and power. For example, supercapacitors exhibit: (i) high specific power due to rapid ion adsorption/desorption near electrode surfaces but (ii) low specific energy since charge storage only occurs within the electrochemical double layer. On the other hand, batteries store energy within the bulk structure of active materials, enabling high specific energy. The rate of energy storage/delivery in batteries is generally limited by solid-state diffusion or phase nucleation kinetics in the active material, resulting in lower specific power than supercapacitors. With these trends in mind, it should be emphasized that the energy/power characteristics of an electrochemical device are also highly dependent on design factors such as material selection, cell format, and electrode architecture.

Sections 1.2-1.5 provide basic overviews of electrochemical energy storage devices where TMOs play critical roles in device operation. The importance of advanced characterization and computing resources on guiding material development, understanding degradation mechanisms, and optimizing system performance is also highlighted.

1.2 Li-Ion Batteries: Basic Principles and TMO Electrodes

Over the last four decades, Li-ion batteries have successfully transitioned from research and development to commercial applications, including portable electronics, electric vehicles, and grid storage. The foundation of this technology is based on cation intercalation reactions wherein Li+ is stored in TMO cathodes and graphite anodes [2]. These intercalation reactions are highly reversible, and today's Li-ion batteries can undergo hundreds or thousands of cycles with minimal chemical and/or structural changes to the active material (see Chapters 9 and 10 for detailed discussion on degradation mechanisms of Li-ion batteries).

The working principles of Li-ion batteries are illustrated in Figure 1.2. During charge, Li+ deintercalate from the TMO cathode (e.g. LiCoO2), transport through the electrolyte, and intercalate into the anode active material (e.g. graphite). To maintain charge neutrality, electrons are simultaneously extracted from the cathode (typically via transition metal oxidation), transported through an external circuit, and inserted in the anode (electrochemical reduction of graphite). During discharge, these processes are reversed, and Li+ ions and electrons are transported back to the cathode. Figure 1.2b shows qualitative cathode/anode voltage profiles as a function of capacity. Commercial Li-ion batteries typically have cell voltages ~3.6 V and specific energies ~200 Wh kg-1, although these values depend on active material selection and cell design.

Commercial Li-ion electrodes are prepared by casting a slurry containing active material (the host material which reversibly stores Li+), electronically conductive carbon additives, and polymer binder onto a current collector (typically Cu for the anode and Al for the cathode). Key electrochemical properties, including operating voltage, reversible capacity, and cyclability, are strongly dependent on the active material's crystallographic structure and transition metal selection. Conventional cathode active materials include: (i) lithium TMOs with layered or spinel crystallographic structures (e.g. LiCoO2 and LiMn2O4, respectively) and (ii) olivine structures containing polyanionic groups (e.g. LiFePO4). A wide range of related compositions containing transition metal substitutions (e.g. LiNixMnyCo1-x-yO2) have been developed to maximize the energy density and cycling stability of TMO cathodes. While most commercial Li-ion batteries contain graphite anodes, some systems also utilize TMO anodes such as Li4Ti5O12. The vast compositional landscape of TMOs explored as Li-ion active materials is detailed in Chapters 2, 3, 5, 7, and 16.

Figure 1.2 (a) Schematic of the working principle of a lithium-ion battery containing a LiCoO2 cathode and graphite anode. (b) Qualitative voltage profiles during charge and discharge.

Source: Belharouak et al. [3].

1.3 Brief History of Lithium-Ion Batteries

In the 1960s, intercalation chemistry was a prominent method used to alter materials' electronic and optical properties [4, 5]. For instance, the electronic conductivity of WO3 can be varied several orders of magnitude by intercalating monovalent cations into the structure [6]. Rechargeable batteries which utilize ion intercalation reactions were first demonstrated in the 1970s by Whittingham [7] at Exxon Corporation. These prototype cells contained a layered TiS2 cathode, lithium metal anode, and liquid electrolyte (e.g. LiClO4 dissolved in a mixture of dimethoxyethane and tetrahydrofuran) [7]. One limitation of this system was the Li metal anode which forms dendrites that can penetrate the separator and internally short-circuit the cell. As an alternative anode, Yazami demonstrated reversible Li+ intercalation in graphitic carbons using a polymer electrolyte in the 1980s [8]. However, in liquid electrolytes these intercalation anodes were hindered by solvent co-intercalation which resulted in graphite exfoliation and electrode degradation during cycling. In 1985, a group led by Yoshino at Asahi Kasei Corporation (Japan) identified petroleum coke anodes [9] which were stable during Li+ insertion/removal, and these anodes were incorporated into Li-ion full cells containing liquid electrolyte. These discoveries ultimately led to the first commercial Li-ion batteries introduced by Sony in 1991. At around the same time, suitable electrolyte solvents (e.g. ethylene carbonate) which do not co-intercalate in graphite were also identified. Compared to disordered carbons derived from petroleum coke, graphitic anodes operate at more negative potentials which enables higher cell voltages and energy densities.

In addition to...