Battery chemical reactions are at the heart of battery design
Well-designed batteries power your equipment efficiently, reliably, and safely for a long lifespan. To achieve this combination of ideals, engineers evaluate certain design parameters, including voltage and power needs, length of duty cycle, temperature conditions, price, and more to guide their design choices.
Battery chemistry is perhaps the most fundamental design choice. At their heart, all batteries are conveniently-packed electrochemical reactions. The ability of a battery to store and discharge electricity depends directly on type of chemical reaction involved.
In this article, we’ll dive into exactly how chemistry influences performance. Then, we’ll look at the properties of some common chemistries.
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Chemistry in batteries affects voltage, energy density, rechargeability, self-discharge, cycle life, and safety
A battery pack is made of one or more packaged battery modules. A battery module consists of electrochemical cells arranged in parallel or series. An electrochemical cell, also called a voltaic cell, has three main parts: the anode, the cathode, and the electrolyte. Together, the anode, cathode, and electrolyte allow an oxidation/reduction reaction to occur.
Oxidation/reduction reactions are about the flow of electrons. When iron oxidizes, iron atoms give up electrons and oxygen molecules gain electrons. The difference between a rusty hunk of metal and a battery is the segregation of these two events. This forces electrons to flow through a circuit to complete the reaction. During discharge, the anode loses electrons, and the cathode gains them. The electrolyte promotes these reactions and allows ions to exist in solution.
During battery discharge, the anode oxidizes and gives up electrons. The electrons flow through the circuit to the cathode, causing a reduction reaction. The electrolyte permits the exchange of ions. During battery charge, the process is reversed. The composition of the anode, cathode, and electrolyte alter many battery properties.
Here are some of the electrical properties that vary depending on battery chemistry:
All voltaic cells of the same chemistry produce the same nominal voltage. This is because voltage is directly related to the favorability of the oxidation/reduction reaction. More favorable reactions produce higher voltages. For instance, lithium-ion chemistries have nominal voltages of about 3.6 V.
Voltage isn’t the only measure of a battery’s worth. You can achieve any voltage and current you need with the proper arrangement of cells in series and parallel. Depending on your needs, that could be heavy, bulky, and expensive. Energy density measures the energy a battery stores relative to its size. Gravimetric energy density is measured in Watt-hours per kilogram (Wh/kg).
It’s a truism that chemical reactions flow in two directions. You can start with reactants and get products, or use the products to re-obtain the reactants. If you apply a reversed voltage and current, you can force the anode to accept electrons and the cathode to give them up, recharging the battery—at least in theory. In practice, it can be extremely difficult to undo some reactions. Primary cells are batteries whose chemistry makes it uneconomical or unsafe to recharge. These are single-use batteries. Secondary cells, also known as rechargeable batteries, rely on easily-reversible electrochemistry. In this article, we’ll focus on chemistries for secondary cells.
Self-discharge and cycle life
In an ideal world, we could charge and discharge batteries over and over again with no loss of performance, or leave them on a shelf for months with no loss of charge. In the real world, voltaic cells suffer from unwanted chemical reactions. While a battery sits on the shelf, spontaneous chemical reactions occur that sap the battery’s charge. This is called self-discharge.
Likewise, when a battery is charged and discharged, unwanted chemical reactions occur that reduce the battery’s ability to store electricity. The number of cycles a battery can be used before it stops performing as intended is the cycle life.
Both the rate of self-discharge and the length of the cycle life depend on battery chemistry. However, they also depend on usage and storage conditions.
Economics, environment, and safety
Finally, take environmental and economic considerations into account. Some battery chemistries are just more expensive than others. Lithium-ion cells are popular and for good reason, but the scarcity of lithium metal makes them expensive.
You should also factor environmental and safety concerns into your economic calculations. Some battery chemistries are highly toxic or corrosive, which can complicate disposal.
Being both affordable and proven, nickel-based chemistries have many applications. In this article, we’ll focus on three major nickel chemistries: nickel-cadmium, nickel-metal hydride, and nickel-iron. Nickel-zinc and nickel-hydrogen chemistries also exist, though they have seen less use. Nickel-hydrogen batteries are notably used on satellites because of their wide range of temperature tolerance.
Nickel-cadmium (NiCd) batteries are rugged, resilient, and provide a long cycle life. Their voltage is also very consistent, so much so that state of charge cannot be determined based on voltage drop like other batteries. NiCd cells can be charged very quickly without compromising safety. Finally, NiCd batteries are some of the most affordable secondary batteries on the market.
However, NiCd batteries come with some downsides. First, while they outpace lead-acid batteries in energy density, they perform worse than other modern battery chemistries. Second, they experience high self-discharge rates.
NiCd batteries experience an interesting but inconvenient phenomenon: the memory effect. The memory effect refers to how, from the outside, NiCd batteries seem to “remember” how deeply they were discharged in the past and only provide that depth of discharge in future cycles. This is caused by the buildup of salt crystals that obstruct the electrode.
Crystal formation causes the memory effect. Crystal buildup obstructs the active portion of the anode. The top image shows normal crystal formation on the anode. The bottom image shows large obstructive crystals. Source: Aero Electric
Finally, and perhaps most importantly, NiCd cells are environmental hazards. Cadmium is highly toxic, and the electrolyte is potassium hydroxide, a very strong base. These hazards make proper disposal of NiCd cells difficult. Though sometimes considered outdated, NiCd cells persist in niche applications like aviation.
Check out these nickel-cadmium battery specifications:
- Nominal cell voltage: 1.2 V
- Energy density: ~45-80 Wh/kg
- Anode: Nickel hydroxide
- Cathode: Cadmium hydroxide
- Electrolyte: Potassium hydroxide
- Self-discharge: ~10%
- Environmental hazards: Toxic, corrosive
Nickel-metal hydride (NiMH) cells are a more-modern iteration of NiCd cells. At first glance, NiMH cells are still very similar to NiCd cells—they still use an alkaline electrolyte like potassium hydroxide, and the cathode is still nickel hydroxide.
The main difference is that NiMH cells trade out toxic cadmium for higher-performance anodes. The exact composition varies—hence the use of “metal”—but all anodes are alloys of rare-earth metals. This substitution increases the cost but makes them more environmentally friendly and provides higher capacity, reduces the memory effect, and reduces temperature sensitivity.
Check out these nickel-cadmium battery specifications:
Nickel-iron cells are uncommon, perhaps because of their high price, low energy density, and high self-discharge. However, they are resilient, long-lasting, and durable in high temperatures.
Check out these nickel-iron battery specifications:
- Nominal cell voltage: ~1.2 V
- Energy density: ~50 Wh/kg
- Anode: Iron
- Cathode: Nickel oxide hydroxide
- Electrolyte: Potassium hydroxide
- Self-discharge: 20-30% per month
- Environmental hazards: Corrosive
Lead-acid batteries are cheap and reliable
Lead acid batteries are workhorses. These resilient and dependable batteries provide power at low cost and experience low self-discharge. Besides the environmental concerns surrounding lead and strong acid electrolytes, the main downsides to lead acid batteries are weight and low energy density. This makes them great for situations where weight is not a major concern, like automobiles, golf carts, forklifts, and uninterruptible power supplies.
Lead acid battery chemistry varies. Lead makes up most of the anode in the form of a grid. However, lead is too soft to support its own weight, so metals like antimony, calcium, tin or selenium are used to improve structural integrity and electrochemical properties.
Check out these lead-acid battery specifications:
- Nominal cell voltage: ~2 V
- Energy density: 30-50 Wh/kg
- Anode: lead (with others)
- Cathode: lead dioxide
- Electrolyte: sulfuric acid
- Self-discharge: 5% per month
- Environmental hazards: toxic, corrosive
Lithium-ion chemistry is at the cutting edge of battery technology
The hottest thing in energy storage today, lithium ion (Li-ion) batteries provide high energy densities and while staying lightweight, making them in-demand for phones, computers, electric vehicles, and more.
Li-ion batteries don’t actually use pure metallic lithium. Like sodium, lithium is highly reactive, and early lithium-metal batteries were prone to catching fire and exploding—a familiar problem for lithium batteries.
Instead, li-ion batteries have cathodes made of ionic lithium compounds. Multiple cathode chemistries exist, including lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), lithium manganese oxide (LiMn2O4), and lithium nickel manganese cobalt oxide (LiNiMnCoO2).
Anodes for li-ion cells consist of a porous material like graphite that sequesters lithium ions and releases them to the electrolyte during battery discharge. Silicone anodes are also promising anode materials. In the future, graphene (one-atom-thick sheets of carbon) may be used as anodes.
There are two major downsides of li-ion chemistries: the cost and the fragility. Lithium can be costly, and lithium mining is an intensive and environmentally harmful process. However, metals like cobalt used in li-ion chemistries drive up battery prices as much or even more than the eponymous lithium.
Besides being expensive, Li-ion batteries require specific operating conditions, being sensitive to overcharge and rapid discharge. Li-ion cells have gained a reputation for fiery or explosive failures. Through proper design and testing, circuit protection, correct use, and the avoidance of damage, these problems can be avoided. But the fact remains that Li-ion batteries are fragile.
This video from EE World Online shows what happens when you remove protective circuitry and ignore safety recommendations for Li-ion batteries:
Check out these lithium-ion battery specifications:
- Nominal cell voltage: 3.6 V
- Energy density: 110-265 Wh/kg
- Anode: carbon, silicon, others
- Cathode: lithium compounds
- Electrolyte: lithium salts in organic solvents
- Self-discharge: 0.35% to 2.5% per month
- Environmental hazards: Flammable organic electrolyte
In battery design, cell chemistry is one of the first and most important choices
From the arrangement of cells to the thermal management system, many factors determine the performance of a battery pack. Battery chemistry is a fundamental choice in battery design and selection.
The composition of the anode, cathode, and electrolyte determine the battery’s voltage, energy density, self-discharge, cycle life, and much more. Of course, battery chemistry interacts in complex ways with other variables. Check out our blog for more information on how temperature influences battery performance.[PD1]
Battery pack design incorporates many variables, of which battery chemistry is just one. Our highly skilled engineers and technicians can design a battery that meets your operation’s specific needs. Contact us to discuss the details project or request a quote to get started on a custom battery pack solution.