Electrochemical Energy Storage

Physics and Chemistry of Batteries

Reinhart Job

• Verlag: De Gruyter
• Jahr: 2020
• ISBN:
  9783110484540
• Auflage: 1
• Thema: Physical Sciences
• Thema: Optics & Light

Starting from physical and electrochemical foundations, this textbook explains working principles of energy storage devices. After a history of galvanic cells, different types of primary, secondary and flow cells as well as fuel cells and supercapacitors are covered. An emphasis lies on the general setup and mechanisms behind those devices to enable easy understanding for students from all technical and natural science disciplines.

1 Introduction

Energy supply and consumption is one of the biggest challenges of modern societies. Focusing on efficient economic growth with a liberal economic order and advanced technologies, the energy demand of the modern rich societies is steadily increasing. Moreover, the emerging countries and economies require an even higher increase of energy demands, if they want to reach the same economic and social level as the rich countries. Considering also the poor countries with their very legitimate aim and right for a much better life for their people, the expected worldwide energy demand will dramatically increase in the future. Although the number of people without access to electricity recently falls below one billion, still hundreds of million people have no access to electric power (in 2020 more than 800 million people). According to the “World Energy Outlook (2019)” of the International Energy Agency, it is expected that the global electricity demand will increase by about 25% per decade for the next two decades. For the most part, the energy supply and, in particular, the electric power supply are still based on fossil fuels like coal, oil and natural gas. In addition, nuclear power also plays a significant role for the electricity supply. On the other hand, it is to annotate that wood is a very important source of energy in poor countries and in poor or remote regions of the world, where burned wood is used as fuel for cooking and heating. Thus, in the poor countries – and not only there – there is a big backlog concerning the countries’ general infrastructure. Hence, infrastructure backlog also drives a strong necessity for an increasing energy supply.

The classical energy resources (coal, oil, natural gas and uranium) face and cause serious problems – already present nowadays and may even intensify in the future. First, the classical energy resources are not sustainable. In the medium-term, shortages will occur. The peak oil phenomenon and the consequences arising from this problem are already evident. Sooner or later, similar shortages – but to a somewhat lesser extend – will be faced with the other classical energy resources. There is still a lot of coal and natural gas on earth; however, thinking about climate change, it would be fatal, if we consume all the carbon-based fossil resources that are still lying in the ground.

Therefore, concerning energy supply with the classical fossil energy resources, one has to speak about climate change and environmental degradation. Climate change and environmental degradation already cause tremendous problems and costs. Continuing the tendency of a steadily increasing energy supply on the base of classical fossil energy resources, the global environmental problems will eventually not be manageable anymore – especially for the poorer countries and probably sooner than generally expected. The upcoming problems, that can be fatal for our modern technological societies as well as for the global climate and environment, flora and fauna, only allow for one solution: The actual classical energy supply has to be transformed – in fact as fast as possible – into a sustainable energy economy based on renewable energies.

The term – or better the concept – sustainability can be traced back to the Club of Rome, that is, to the study The Limits of Growth published in 1972, where the necessity to reach a sustainable ecological and economic stability was emphasized. The study simulated the consequences of interactions between the environment and natural resources of the planet Earth and human systems upon a steadily increasing population and economy. In particular, the consumption of nonrenewable natural resources was taken as a variable into account – besides four other variables, that is, population growth, food production, industrialization and environmental pollution. The study first gave insights into the limited resilience of our planet Earth upon steadily growing population numbers and continuing economic growth; that is, the constraints were disclosed that a limited planet put on population numbers and human activities.

Nowadays, the term sustainability comprises three categories that are integrated in the model of the so-called triple bottom line (abbreviated TBL or 3BL), that is, an accounting framework with an ecological, a social and an economic or financial part, respectively (Figure 1.1). Sometimes, one also speaks about the three pillars of sustainability. The concept is based on the term sustainable development that was defined in 1987 by the Brundtland Commission of the United Nations. The ecological pillar defines the requirements and goals with regard to nature, ecology, environment and climate. The social pillar defines the development of future societies that satisfy all people in all regions of the world, promoting a peaceful and worth living global community. The economic pillar establishes a durable – that is, sustainable – base for the production and acquisition of products avoiding the exhaustion of natural resources.

Figure 1.1: Model of the triple bottom line or the three pillars of sustainability: sustainability comprises three categories, that is, an ecological part, an economical part and a social part. Sustainability can be achieved if the ecological, the economical and the social pillars are respected, that is, in the overlapping areas in the center of the graph.

Speaking about a sustainable energy sector, several significant thematic key aspects have to be taken into account, that is, usable energy and reasonable energy supply and resources, further development and optimization of technological concepts, systemic challenges and social transformations, consumption and efficiency, transportation and storage and so forth. Usable energy resources are manifold and comprise the classical fossil energy resources (coal, gas, oil, uranium, etc.) and prospective energy resources like hydrogen or methane hydrates as well as the renewable energies. If we do not want to concentrate on nuclear energy, with regard to climate change, renewable energies are the only solution for a sustainable energy supply.

Besides solar and wind energy, the renewable energies also comprise hydropower, tidal power and ocean wave energy, biomass energy and geothermal energy. Hydropower is well developed; however, in densely populated countries, a further expansion of hydropower is difficult and often impossible. Tidal power requires appropriate geographic locations, and ocean wave energy does not play a big role up to now (and probably also not in the future). The application of geothermal power is promising, but is not yet a well-developed technology. Moreover, the usage of biomass is not convincing for several reasons (erosion of land, ethically questionable, etc.).

Solar and wind energy exhibit a great potential for the medium- and long-term energy supply in the future. The solar and wind power technologies are well developed; however, energy storage is required due to variable energy yield. Hence, according to an increasing fraction of wind and solar energy supply, a reliable energy distribution also requires increasing storage capacities. Large battery systems, especially rechargeable battery systems, are an option to counteract power grid instabilities. Moreover, a growing trend towards decentralized energy supply and grid structure – that is, so-called microgrids – requires energy storage, too. Microgrids are a localized group of electrical power sources and loads that are usually connected to a conventional wide area synchronous grid and that are synchronous with this macrogrid. However, microgrids can be also operated in an island mode, that is, if it is disconnected from the macrogrid. Microgrids can integrate various distributed energy sources in an efficient way, especially renewable energy sources like wind or solar power. For microgrids, energy storage is essential, since the power quality has to be ensured – including frequency and voltage regulation, smoothing of the electrical output of the renewable energy sources and, last but not least, to provide a backup power for the system. Battery systems can highly support such storage requirements.

Considering energy storage with regard to electrical energy supply at large, a number of particular duties and challenges have to be taken into account depending on the actual situation. In general, electric energy supply has to be secure and guaranteed. Electrical energy for peak demand as well as for the compensation of the load fluctuations in the grids has to be kept at hand. In addition, energy storage can be useful for an efficient energy conversion and consumption. Moreover, electrical energy storage can even support climate protection and sustainability.

Thus, electrical energy storage distinctly varies depending on the particular purpose and application. Important parameters are, for instance, the amount of energy that has to be stored, the necessary storage dynamics and power gradients that are possible during charging and discharging. Other important factors are the lifetime of storage devices or the number of storage cycles (charging and discharging cycles) that are possible. Moreover, location dependencies, stationary or mobile operation have to be considered, too. Finally, it is also of consequence and relevant, if the electrical energy storage is considered for short-term, medium-term or long-term operation or even for seasonal storage applications.

In the last decades, electrochemical energy storage and conversion, that is, batteries, became more important for electrical energy storage. Mobile communication and computing have driven the development of powerful battery types with high capacities (lithium-ion batteries). Moreover, due to the advent of electromobility, that is, the transformation of gasoline- or diesel-driven cars to hybrid and electric cars, it can be highly expected that the demand for batteries will dramatically increase. Finally, large batteries or battery systems for electrical energy storage for a reliable renewable energy supply (solar and wind power) will also strongly increase in the medium term. In parallel, the expectations of the customers with regard to battery performances, durability, energy content, weight and so on are also steadily increasing.

Batteries can be divided into two groups, that is, into primary batteries that are not rechargeable and into rechargeable secondary batteries. Fuel cells can be designated as tertiary batteries, but they are not a subject of matter in this book. Batteries are galvanic cells that spontaneously transform chemical energy into electroelectrical energy. In particular, they operate as a voltage source. The principle mechanisms of batteries are based on redox reactions (reduction–oxidation reactions), where the oxidation and reduction reactions occur in two spatially separated half-cells. The half-cells are internally connected by an ion conductor and externally by electrical wiring a current can flow that can be used for some application as desired. The detail will be discussed in Chapter 3. In general, a galvanic cell can be constructed by any combination of two different electrodes (i.e., an anode and a cathode) and an appropriate electrolyte in between; and it works as a battery. Upon discharge in the anodic half-cell, an oxidation reaction occurs, and at the cathode, a ...