Polymer-based Solid State Batteries

Daniel Brandell, Jonas Mindemark, Guiomar Hernández

• Publisher: De Gruyter
• Year: 2021
• ISBN:
  9781501514906
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

1 Polymer electrolyte materials and their role in batteries

1.1 Battery growth

This current era is experiencing a tremendous growth in the interest and application of batteries. From being household items bought in supermarkets, batteries are rapidly becoming larger and larger in size, and thereby also more and more costly and complex to manage. This is connected to the world clearly entering a period of electrification. Electromobility of vehicles from scooters to electric flights requires high-performance energy storage, and intermittent energy sources from solar panels and wind parks need high-quality storage with a high energy efficiency. With a shortage in energy supply, energy storage units with poor conversion efficiency will have difficulty to compete with batteries, where the energy output is largely equivalent to the energy input. While large-scale storage in the grid and small-scale storage in Internet-of-things devices are rapidly growing in demand, today’s exponential growth in the demand of batteries is primarily driven by the transport sector, and especially due to the similarly exponential growth in electric vehicles (EV). This trend is foreseen to dominate during the next decade [1].

For a high versatility of batteries, that is, to have the ability to use them in a wide range of products and applications, they need to be able to supply a vast amount of energy per gravimetric and volumetric unit. These concepts are known as specific energy (Wh kg^–1) or energy density (Wh L^–1). The same is true for the power density of batteries, equivalent to the energy delivered per unit of time and either weight or volume (W kg^–1 or W L^–1). Since batteries can be connected in series or parallel in an electric circuit, it is not difficult to obtain a high energy or power storage capability irrespective of battery chemistry, but if the energy density is low it will result in very big or bulky battery packs. Therefore, it is vital to maximize the energy content per gravimetric and volumetric unit, in particular for mobile application where the penalty is strong for extra weight and volume.

The specific energy E_sp of a battery is determined by two factors: the specific capacity Q/m (Ah kg^–1) and the voltage U (V):

(1.1)$E_{\mathrm{s}\mathrm{p}}=\frac{U\times Q}{m}$

In a battery, where the released energy is determined by redox reactions taking place in the battery electrodes, the voltage describes the potential difference between the battery electrodes – the driving force for the battery reaction – while the specific capacity is equivalent to how many times this electrochemical reaction can occur. One can make an analogy with driving in a nail with a hammer: the voltage corresponds to the force of hitting the nail, while the capacity corresponds to how many times the nail is hit.

This partly explains why the Li-ion battery (LIB) technology has become dominant among secondary (rechargeable) batteries. It is relatively simple to find LIB electrode materials with a large voltage difference, while these materials can also store a lot of lithium, thereby providing high voltage – in fact, the highest theoretical voltage of all elements due to the low reduction potential of Li – at the same time as providing high capacity. Moreover, the small size and lightweight of Li⁺ ion leads to high-energy-density electrode materials where it is relatively easy to find host structures where the Li⁺ ions can jump in and out during battery charge and discharge. This, in turn, leads to batteries that can cycle for an extensive amount of cycles. Other commercial (lead–acid, nickel–cadmium, and nickel–metal hydride; see Fig. 1.1) and largely noncommercial (Na-ion, Mg-ion, Ca-ion, and Al-ion) battery chemistries often fail in one or several of these categories: compared to LIBs, they do not provide the same energy density or the same cycle life. LIBs therefore have, despite shortcomings in terms of cost and safety, grown to become a very useful and versatile battery type.

Fig. 1.1: Gravimetric and volumetric energy densities for commercial and noncommercial battery chemistries. Illustration taken from the Battery2030 + roadmap “Inventing the Sustainable Batteries of the Future. Research Needs and Future Actions.”

1.2 The Li-ion battery and its electrolyte

Considering the current prominence of the LIB and its importance for the ongoing societal electrification, there will also be a natural focus on LIB chemistries throughout this book. Like all batteries, LIBs function through parallel chemical redox reactions at the two electrodes: oxidation at the anode and reduction at the cathode (Fig. 1.2). During discharge, the electrons liberated by the anode oxidation are spontaneously transported in an outer circuit over to the cathode side, where they are accepted in the reduction reaction. The electronic current this gives rise to can then be used to produce electric work. In an LIB, the anode is often graphite with Li⁺ ions intercalated between the graphene sheets. During the oxidation reaction, Li⁺ ions leave the electrode and travel into the electrolyte, and graphite thereby gives up one electron:

$\mathrm{L}\mathrm{i}{\mathrm{C}}_6\to \mathrm{L}{\mathrm{i}}^{+}+{\mathrm{C}}_6+{\mathrm{e}}^{-}$

The cathode, in turn, normally consists of a transition metal oxide. The transition metal ions are reduced when Li⁺ ions are inserted – intercalated – into the host structure from the electrolyte. One common example, and the predominant cathode material in cell phone and laptop batteries, is LiCoO₂ (LCO):

$\mathrm{C}\mathrm{o}{\mathrm{O}}_2+\mathrm{L}{\mathrm{i}}^{+}+{\mathrm{e}}^{-}\to \mathrm{L}\mathrm{i}\mathrm{C}\mathrm{o}{\mathrm{O}}_2$

There is a range of other cathode materials employed in commercial LIBs, for example, LiFePO₄ (LFP) and LiNi_xMn_yCo_zO₂ (NMC). LFP is by comparison often considered more sustainable (due to that Fe is common in the Earth’s crust) and is useful for high-power applications with extensive cycling but, on the other hand, has a rather low operating voltage (ca. 3.5 V vs Li⁺/Li). NMC, which exists in several different compositions (i.e., the values of x, y and z in LiNi_xMn_yCo_zO₂ can be varied), is dominating for EVs primarily due to its high energy density.

The role of Li⁺ ions in this process is thus to charge compensate in the two different redox reactions that occur spontaneously in the anode and cathode, and which take place due to the thermodynamic driving force of the system. During charging, when energy is supplied to and stored in the battery, the reverse processes occur: Li⁺ is inserted into the graphite anode, and correspondingly deinserted from the cathode material, while graphite and transition metals are reduced and oxidized, respectively. This means that an effective medium needs to transport Li⁺ ions between the two electrodes during battery operation. This is the role of the electrolyte, which is at the focal point of this book. The electrolyte consists of a salt dissolved in a solvent; for LIBs this is a lithium salt. While the electrolyte does not store any energy in itself, it plays a vital role for current transmission in the battery cells. At the same time, the electrolyte contributes additional materials, weight and volume to the system, and thereby influences energy density, cost and sustainability in a negative way. Ideally, an electrolyte should contain as little and as simple materials as possible but still provide useful ion transport properties. From a user perspective, the battery electrolyte is a necessary evil.

The final battery cells can have different forms depending on their intended use, and these also have their pros and cons. The most common forms are cylindrical cells, prismatic cells and pouch cells. All of these, however, are based on a two-dimensional design with two electrode sheets facing each other, and the electrolyte is located in between, immersed in a separator which prevents the electrodes from making contact and thereby short-circuiting. In cylindrical and prismatic cells, these sheets are rolled or wound up. In large-scale commercial applications, such as vehicle batteries, several cells are then organized into a module, and the modules in turn placed into a battery pack. The battery pack can contain hundreds of cells and is organized for efficient power transmission and cooling of the cells during operation.

As also shown in Fig. 1.2, there are a number of different electrochemical processes that are necessary to run in parallel for the battery to work. In this context, it is of importance to acknowledge that the electrodes in the battery are composites, generally with three major components: (i) the active material undergoes the electrochemical redox reactions; (ii) an electronically conductive carbon additive; and (iii) a polymeric binder that keeps the electrode structure together. The additive and binder, similarly to the electrolyte, contribute to deadweight in the battery, and are therefore generally reduced to a few percent of the electrode content. These components form a porous mixture, and the electrolyte is immersed into voids between the particles. The major processes that need to take place for the battery to operate are thus:

• the electrochemical oxidation and reduction processes, which occur in the active material particles;
• electronic transport from the redox centers and out to the battery current collector, facilitated by the carbon additive;
• solid-state transport of lithium ions from the center of the active material particles, out toward the surface and into the electrolyte, and vice versa for the reverse process;
• diffusion and migration of lithium ions from the electrode particle surfaces in the electrolyte-filled voids of the electrode, through the bulk electrolyte in the separator, and into the pores of the counter electrode.

Fig. 1.2: Basic structure of the Li-ion battery cell, its components and fundamental processes. The white spheres illustrate the active anode and cathode materials, the black spheres the carbon additives used for electronic wiring, the blue strings the electrode binder and the green spheres lithium ions. The inset shows the electrochemical redox rea...