Battery Technology/Chemistry

Lead- Acid:

Globally accounting for about 40-45% sales, lead-acid batteries find numerous applications. Lead-acid batteries use lead dioxide (PbO) as the positive active material and lead (Pb) as the negative active material immersed in sulfuric acid which acts as electrolyte. Main advantages of lead-acid batteries include: easy estimation of the SoC, since it has a direct relation with the specific density of the electrolyte; good charge retention, suitable for intermittent recharging; cell components are easily recyclable; low cost, and high OCV cell voltage.

This battery technology also has several limitations, for example: most lead-acid battery types require periodic maintenance, since they tend to lose water while in operation, so it needs to be replenished; lead is heavy and therefore these batteries tend to be heavier than other technologies, which limits their portability; relatively low cycle life, although some types can achieve up to 2000 cycles, most used ones have a considerably lower number; the thermal runaway issue in this battery technology, when the internal heat generation from the charging current flowing through the resistivity components; among others.

In the transportation sector, modified lead-acid batteries with high cycle life and high DoD capabilities have been used for decades in battery-powered trucks, forklifts, elevating trucks, and other vehicles for internal factory transportation as well as leisure vehicles, such as golf carts. However, due to technology developments in other battery technologies, specially Li-ion, the use of lead-acid batteries in most vehicles sold today has been limited to start, lighting and ignition (SLI) operations.

Nickel-Cadmium (Ni-Cd):

Manufactured in two main versions: unsealed batteries, where the electrolyte and gas can escape through a vent; and a fully sealed battery which do not require refilling the electrolyte with water. The first version is often used in traction applications as a flooded battery, while the latter is used as a portable power source. Nickel-cadmium batteries are considered to be very reliable and to have a long cycle life.

One important characteristic is that they exhibit voltage suppression, also known as memory-effect. This means that the battery can only provide with the capacity that was used during the repeated charge/discharge cycles before. Due to this, the battery should be fully discharged before recharging again, to avoid losing storage capacity. In consequence, this battery technology is not suitable for applications that do now allow complete discharge.

Some of the advantages of this battery technology include: longer life time, lower maintenance, and broader temperature operating range than lead-acid batteries; low self-discharge rate and high sturdiness, which makes them ideal for heavy duty applications. Its main limitations include its memory-effect and the fact that cadmium is an environmentally hazardous material, so the final disposal of these batteries is a major issue .

Nickel-Metal Hydride (Ni-MH):

Nickel-metal hydride (Ni-MH) batteries are very similar to nickel-cadmium batteries, because they share the same positive electrode and the same electrolyte, however, for the negative electrode, hydrogen is used instead of cadmium. Furthermore, the negative electrode corresponds to a fuel cell electrode rather than to a conventional battery. In the charged state, hydrogen remains as gaseous gas within the cell, therefore nickel-metal hydride batteries must be hermetically sealed. In the discharged state, the hydrogen is absorbed by the nickel hydroxide.

Another difference with Ni-Cd batteries is that in Ni-MH batteries, the cell reaction exothermic and therefore, the internal temperature of Ni-MH cells rises during operation. There are several advantages for Ni-MH batteries over Ni-Cd ones. First, Ni-MH do not pose the same environmental hazards as Ni-Cd batteries do. Additionally, Ni-MH batteries have higher energy density and specific energy, and most importantly, they do not exhibit the same level of memory-effect as Ni-Cd batteries do.

There are, however, several limitations for this battery technology, for instance: overcharging can overheat the battery and release hydrogen that would pose a serious fire hazard. Therefore, complex charging circuitry is required; when discharged at high-current levels, its life time reduces significantly (200–300 cycles); higher self-discharge rate than Ni-Cd; among others .

Ni-MH batteries have been very popular in electric vehicles, such as the Toyota “RAV-EV” as well as in hybrid vehicles, like the popular “Prius.” This has helped Ni-MH batteries gain an important position in the transportation sector and has displaced other technologies, like lead-acid and Ni-Cd. Today, its main competitor are Li-ion batteries.

Lithium-Ion (Li-Ion):

It is the most popular technology for portable electronics, such as mobile phones and portable computing. Lately, thanks to the cost reduction in lithium-ion batteries manufacturing and high incentives towards clean transportation, this battery technology is becoming very popular among electric vehicle manufacturers. The most frequently used lithium-ion battery type is the lithium-nickel-manganese-cobalt-oxide. These batteries use a new chemistry commercialized since 2004. The cathode of these batteries is made of mixed oxide.

In the batteries, lithium ions move from the negative electrode through an electrolyte to the positive electrode during discharge, and back when charging. Li-ion batteries use an intercalated lithium compound as the material at the positive electrode and typically graphite at the negative electrode. The batteries have a high energy density, no memory effect (other than LFP cells) and low self-discharge. They can however be a safety hazard since they contain a flammable electrolyte, and if damaged or incorrectly charged can lead to explosions and fires.

Chemistry, performance, cost and safety characteristics vary across LIB types.

Handheld electronics mostly use lithium polymer batteries (with a polymer gel as electrolyte) with lithium cobalt oxide as cathode material, which offers high energy density, but presents safety risks, especially when damaged. Lithium iron phosphate (LFP), lithium ion manganese oxide battery ( LMO), and lithium nickel manganese cobalt oxide (NMC) offer lower energy density but longer lives and less likelihood of fire or explosion. Such batteries are widely used for electric tools, medical equipment, and other roles. NMC in particular is a leading contender for automotive applications.

Research areas for lithium-ion batteries include life extension, energy density, safety, cost reduction, and charging speed, among others. Research has been under way in the area of non-flammable electrolytes as a pathway to increased safety based on the flammability and volatility of the organic solvents used in the typical electrolyte. Strategies include aqueous lithium-ion batteries, ceramic solid electrolytes, polymer electrolytes, ionic liquids, and heavily fluorinated systems.

Flow Batteries:

A flow battery, or redox flow battery (after reduction–oxidation), is a type of electrochemical cell where chemical energy is provided by two chemical components dissolved in liquids contained within the system and separated by a membrane. Ion exchange (accompanied by flow of electric current) occurs through the membrane while both liquids circulate in their own respective space. Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 to 2.2 volts.

A flow battery may be used like a fuel cell (where the spent fuel is extracted and new fuel is added to the system) or like a rechargeable battery (where an electric power source drives regeneration of the fuel). While it has technical advantages over conventional rechargeables, such as potentially separable liquid tanks and near unlimited longevity, current implementations are comparatively less powerful and require more sophisticated electronics.

The energy capacity is a function of the electrolyte volume (amount of liquid electrolyte), and the power is a function of the surface area of the electrodes. Flow batteries can be rapidly "recharged" by replacing the electrolyte, they can be used for applications where the vehicle needs to take on energy as fast as a combustion engine vehicle

Flow batteries are normally considered for relatively large (1 kWh – 10 MWh) stationary applications