IEEE Electrification Magazine - September 2015 - 15

The effective
management of
PV battery storage
requires a strong
interaction with the
weather forecast.

offers a system the ability to significantly reduce the impact of variability in generation.
Batteries played a principal role in the evolution of energy storage. Significant research effort has resulted in a more sophisticated knowledge of
battery chemistry, which has consequently given us a rapid reduction in
cost while addressing major concerns about reliability and stability.

Evolution of Battery Technology
In the last three to four decades, improvements in the chemistry of lithium-ion (Li-ion) cells have allowed the rapid development of electronic
products ranging from battery-operated radio transmitters to power grid
storage systems and battery-powered electric vehicles.

Charging and Discharging
The principle of operation of Li-ion cells has been well explained in the literature (see
Yoshio et al., 2009; Wkihara and Yamamoto, 1998; and Pistoia, 2014). Each cell is composed of four components: a positive electrode, a negative electrode, an electrolyte, and a
separator in between them. The positive electrode is made from a metal oxide material
[such as lithium cobalt oxide (LiCoO2)], a material with a tunneled structure [such as lithium manganese oxide (LiMn2O4)], or a material with an olivine structure [such as lithium
iron phosphate (LiFePO4)]. The negative electrode is carbon (graphite), and the electrolyte varies from one type of battery to another. The separator prevents physical contact between the anode and cathode, while facilitating ion transport in the cell.
During the charge process of the battery, the lithium-based positive electrode releases some of its lithium ions, which move through the electrolyte to
reach to the negative electrode and remain there. The battery stores energy
during this process. When the battery is discharging, the lithium ions move
back across the electrolyte to the positive electrode, producing the energy
required by the load connected to the battery. In both cases, electrons flow
in the opposite direction of the ions around the outer circuits. The electrons do not flow through the electrolyte as it tends to be an effective
insulating barrier for them.
The movement of the ions (through the electrolyte) and electrons (around
the external circuit, in the opposite direction) are interconnected processes,
and if one of them stops, the other also stops. If ions stop moving through
the electrolyte because the battery completely discharges and the electrons
cannot move through the outer circuit either, the power is lost (Figure 2). Also,
at no-load conditions, the electron flow stops, and, consequently, the flow of
ions forces the battery to stop discharging (Oswal et al., 2010).

The Search for the Right Materials
Lithium is the most electropositive element and is lightweight. Lithium ions
can move faster in lighter materials. Getting to the point where we are today
with respect to the evolution of Li-ion batteries required the development of
technologies based on new anodes, cathodes, and electrolytes.
Lithium-metal anode primary batteries based on nonaqueous electrolytes such as propylene carbonate-lithium perchlorate and lithium negative electrodes were developed in the
early 1970s. In 1973, Matsushita introduced a lithium-carbon monofluoride primary cell. In
1975, Sanyo was the first to commercialize lithium-manganese dioxide cells; they were not
rechargeable but were able to produce 3.5 V. The basic usage of these cells was for cameras
and memory backup applications. In the following years, efforts were made to convert lithium cells into rechargeable cells with high energy density. Different materials were studied
without achieving any competitive advantage, except by the polyacene battery, where the
main usage was in the production of coin cells for memory backups (Yoshio et al., 2009).
Between 1973 and 1976, Stanley Whittingham at Exxon developed a novel material for
the cathode based on titanium disulfide and started the principle of Li-ion rechargeable
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Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2015