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Basics of Batteries

  • Batteries are electrochemical devices used to store electrical energy

  • Chemical reactions in batteries cause electrons to flow from one electrode to another through an external wire, while ions move through the battery via the electrolyte

  • To recharge a battery, electricity is applied to the battery to make the electrons and charged particles (ions) move in the opposite direction of their natural flow

  • A large variety of battery types exist, differing in their electrode and electrolyte materials

  • Lithium ion batteries have unlocked the possibility of rechargeable portable electronic devices and will play a key role in electricity storage going forward

  • Batteries, like most human-developed technologies, face challenges of efficiency, lifetime, waste, and environmental and social impact

Since electrical energy is a continuous flow that must be used as soon as it is generated, it cannot be stored in electrical form. Rechargeable batteries are a way to store electrical energy by converting it into chemical potential energy.

Types of batteries and their applications

Batteries can be made out of a wide range of different materials, each with advantages and drawbacks suited to different applications. Different battery technologies have been developed over time and are described briefly below. As we move toward an electrified energy infrastructure, lithium ion batteries are expected to play a key role for household storage, electric cars, and portable devices.

Alkaline batteries are small, cheap, usually single-use batteries used in many household devices, including flashlights, toys, and smoke detectors. Alkaline batteries have a long shelf-life, and they don’t use toxic heavy metals such as lead or cadmium. A chemical reaction between zinc and manganese oxide transfers electrons and ions within the battery. They’re called “alkaline” because the electrolyte used is potassium hydroxide (KOH), which is a very basic or “alkaline” solution.

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Lead acid batteries are one of the oldest rechargeable battery technologies, invented in 1859. They are often still used in cars to power the electronics and start the engine because they can discharge a lot of energy quickly. These batteries transfer protons and electrons in the chemical reaction between lead and sulfuric acid.

Nickel cadmium (Ni-Cd) batteries were the first portable rechargeable batteries, used in power tools and rechargeable AAs. The chemical reactions between nickel oxide and cadmium are used to generate electrical energy in these systems. However, the toxicity of cadmium (particularly upon disposal) incentivized the development of nickel metal hydride (Ni-MH) batteries, which have mostly replaced Ni-Cd. An advantage of nickel metal hydride batteries is that they don’t use the heavy, toxic metal cadmium, and they have a longer shelf life by limiting unintended and wasted battery discharge.


Lithium ion (Li-ion) batteries are a newer technology that power all of our smartphones, laptops, and most new electric cars. They are one of the key technologies that unlocked our current digital age. They are light and compact, and have much higher energy density than the other batteries listed above. Lithium ion batteries can vary widely, with a range of different materials used in the electrodes and electrolyte. The common feature is that they use lithium ions (Li+), the smallest and lightest positive ion after hydrogen (H+). Lithium ions can be stored up within electrode materials and then released. Because of their small size, many lithium ions can be stored in the material, leading to the high energy density of the batteries.

Energy density in batteries

Each of the battery types above has a range of parameters that can make batteries more energy dense. Here is a comparison of the energy density (by weight) vs. volumetric energy density (by volume) for rechargeable batteries. As shown in the graph, the Li-ion is by far the most energy dense battery in this series.

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Battery fundamentals

Batteries store energy in the form of chemical potential energy. This stored energy can then be released as electricity when it is needed for use.  In the case of rechargeable batteries, electrical energy can reverse the chemical process that releases a battery’s energy, and instead store up more chemical potential energy in the battery for future use.

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Batteries have two different electrodes, or materials, that are connected to a circuit to generate electricity. The electrodes are the anode (formally called the “negative” electrode) and the cathode (formally called the “positive” electrode). They are separated by a chemical material called the electrolyte. When the electrodes are connected by a wire, electrons are released from the anode and move through the wire to the cathode. Within the battery, ions move through the electrolyte to complete the circuit and balance the charge.


During charging, external inputs of electricity force electrons in the direction they wouldn’t naturally flow in, causing ions to build up in one of the electrodes. When it is time to release the chemical potential energy through discharging, the ions flow back to where they would prefer to be, causing electrons to move in the process, generating an electric current.

In single use batteries, ions and electrons only flow one direction and once the battery is fully discharged, it cannot be used anymore. In rechargeable batteries, however, external electrical energy can be applied to make electrons and ions move back in the non-favorable direction to store electrical energy. Each charge cycle of a rechargeable battery loses some efficiency, however, causing the storage capability to degrade over time. This is why your phone or computer battery life gets worse as your device ages.

Emerging battery technologies

Currently, batteries are most useful for small-scale, portable storage applications. With growing demands for energy storage to balance variable renewable energy from solar or wind power, batteries may also be used for larger-scale grid storage. Below are just a few battery technologies in development.

Redox flow batteries use tanks of electrolyte solutions rather than material electrodes as the anode and cathode. When the two different solutions meet in the reaction cell, electrons and ions are transferred to generate electrical current. The advantage of this system is that the capacity of the battery scales with the size of the electrolyte tanks, and they are therefore most applicable at industrial scale with large tanks. However, they can also be scaled to meet different capacities on a non-industrial size.

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Another battery technology in development is sodium ion batteries. These operate similarly to lithium ion batteries. They are less efficient and energy dense than lithium, since sodium is larger than lithium, but sodium is much cheaper and abundant than lithium.

With the growth of electric cars, portable devices, and an interest in household battery storage, lithium ion battery technology is still being improved. Specifically, new anode and cathode materials are being developed that have better capacity and stability with lower material costs.


Batteries, like most human-developed technologies, face challenges of efficiency, lifetime, waste, and environmental and social impact.


Rechargeable batteries lose efficiency over time, and alkaline batteries are single use. The limited lifetime of batteries necessitates consideration of recycling or disposal methods. Most used batteries currently end up in landfills, and their components can pollute the nearby environment and groundwater supplies.

The rapid expansion of electrical devices that use batteries will escalate demands for the materials needed to manufacture them. Some battery technologies use toxic materials such as lead and cadmium. Mining cobalt and nickel (metals commonly used for electrodes) and lithium also can have negative implications on the land they are extracted from, water supplies, people in those areas, and more.


An estimated 50-75% of the world’s lithium, for example, is found in the “Lithium Triangle” region of Bolivia, Argentina, and Chile. Here, there is a large population of Indigenous peoples who have reaped few benefits from mineral extraction on their lands. In addition, the high demand for water in the mining process in this dry climate has had significant downstream effects on agriculture.

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As battery production expands rapidly in the coming years, these environmental and ethical impacts must be taken into consideration.

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Questions for deeper thinking

  • Why might it be beneficial to have a lot of different types of batteries for different purposes? Consider material abundance, efficiency, and supply chains.

  • How do the costs of battery material extraction fit into the issue of planetary health and clean energy (such as solar and wind) versus fossil fuels debates?

Sources and further reading

Page last updated:  November 20, 2022​

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