Batteries are galvanic cells in which chemical energy from electrochemical reactions is converted to electrical energy that can be harnessed externally. They contain two chemically different electrodes sandwiching an ion-conducting electrolyte phase. At open circuit, a measurable electrical voltage develops between these electrodes due to the propensity for chemical reactions to occur. Upon electrical connection to a load, chemical reactions occur and ions travel across the electrolyte. A spontaneous flow of electrons occurs in the external circuit from the electrode with the more negative potential to the more positive electrode, and can therefore be used to power a load such as a device (see Figure 7.1). If the battery is rechargeable, the reverse reactions can be induced with an input of energy. Depending on the type of reactions that occur, for example displacement reactions or insertion reactions [6], the discharge behavior of the battery will vary (Figure 7.2), resulting in different cell
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Figure 7.1 Battery schematic.
(a) (b)
potential outputs (V) with respect to the amount of charge (q) extracted from the battery, and from this relationship the amount of useful energy (E) obtained from the battery can be determined from the following expression:
E = JV dq (7.1)
The energy is typically reported in units of joules (J) or watt-hours (Wh). The total amount of charge extracted from the battery, or how much that can be stored in the cell, is known as the capacity typically reported as amp-hours (Ah) or coulombs (C). The maximum power, measured in watts (W), that can be drawn from the battery is dependent on the kinetics of the system. This system can be limited by the summation of impedances in the cell, including the charge transport in the electrodes and electrolyte, the rates at which reactions occur, and interfacial resistances. The power behavior of a battery can also be characterized by its rate performance, and is evaluated by the time (in hours) it takes to deplete a device of its maximum storage capacity (C). For example a battery that takes 10 hours to completely drain was discharged at a C/10 rate, while a quick discharge of 2C means the battery was depleted in a half hour. Note that all the quantities listed can be normalized with respect to weight, volume, and footprint area. For example, electrochemical storage energy is primarily quantified in terms of the specific energy (Wh/g), the amount of energy stored per unit volume (Wh/cm3), and the energy stored per unit footprint area occupied on a substrate (Wh/cm2). Note that when quantifying the theoretical performances of electrochemical materials, metrics normalized with respect to weight and volume are customarily used; however, when comparing microdevices, the constrained unit is most often the footprint area occupied on a substrate, and to compare across many fabrication methods, materials, and device configurations, areal metrics are pertinent.
The chemistries and relative amounts of the battery components will determine its operating voltage and energy storage capacity, and along with the cell geometry and processing, will also influence the maximum power accessible. In addition to these properties, other performance metrics include the battery’s lifetime (or for rechargeable batteries, its cycle life) as well as its safety and cost, all of which depend on its inherent materials properties, such as their stability and compatibility. Though batteries are straightforward conceptually, there have been numerous chemistries, geometries, and processing methods proposed.
