Lithium Iron Phosphate (LiFePO4) Battery Energy Density

In the battery world, energy density refers to the amount of electrical energy stored in a battery per unit volume. It is an important parameter that helps in battery comparison. It is expressed in Watt hours per liter (Wh/L).

The energy density of LiFePO4 often is a point of discussion among professionals, battery experts, and amateur buyers. LiFePO4 is one of the newest and most advanced battery technologies. Industries are quickly adopting it. As such, it is important to know what is the energy density of LiFePO4 batteries.

Comparing it against other lithium-ion batteries can provide a perspective on whether to switch to LiFePO4 or not.

What is LiFePO4 Battery?

LiFePO4 stands for lithium iron phosphate. The LiFePO4 battery is an improvement over conventional lithium-ion rechargeable batteries. Lithium Iron Phosphate is the cathode material. The anode is made of graphite. LiFePO4 has replaced lead-acid and lithium-ion batteries in every deep-cycle application. Some common advantages of these batteries over other LiFePO4 batteries are:

• Longer lifespan
• Non-toxic nature
• Higher Cycle life
• Better depth of discharge
• Higher operation range
• Overcharge protection

Understanding the Energy Density of a Battery

The energy density is indicative of the power of a particular sized battery. This parameter is important for batteries in mobile applications where weight can play an important role. For stationary applications, battery capacity and cost per KWh can be a more useful comparative factor.

The importance of energy density can be understood by taking the case of electric vehicles. Electric vehicles extensively use LiFePO4 deep cycle batteries over lead acid counterparts. This is because LiFePO4 batteries have a four times higher energy density. Therefore, the weight of the LiFePO4 battery will be one-fourth to that of a comparative lead acid counterpart. Of course, there are other benefits at play as well like the longer battery life.

Types of Battery Energy Density

For electrical batteries, two types of energy density representations are used:

Specific Energy Density

It is also known as the gravimetric energy density. It means the energy stored in the battery per unit mass. It is written as Wh / kg. It is the standard representation of energy density unless written otherwise. Specific energy density is a reflection on how much a particular capacity battery will weigh.

Volumetric Energy Density

Volumetric energy density refers to the amount of electrical energy stored in the battery per unit volume. These are expressed as Watt hours per litre (Wh/l). The volumetric energy density of lithium cells is a reflection of how much space the cells will occupy.

What is the Theoretical Energy Density of LiFePO4 Battery?

The energy density of LiFePO4 batteries typically falls from 140 Wh/L (504 kJ/L) to 330 Wh/L (1188 kJ/L). Many high-range batteries can offer an energy density of up to 325-330 Wh/L.

An important thing to note is that these energy density values are just the theoretical value on paper. The actual energy density of the LiFePO4 cells will vary based on various parameters discussed in the latter sections.

LiFePO4 Energy Density vs Power Density

Many people confuse the energy density of li-ion batteries with its power density. Even some professionals mistake these two terms to be synonymous. However, there are crucial differences between the two.

The difference between energy density and power density is the same as the difference between energy and power. Energy density represents the amount of energy stored in the battery. Power density represents the capacity of the battery to discharge that energy.

A high energy density does not imply a high power density. If energy density is high but power density is low, the battery will be able to power a reasonable number of appliances for a very long time. If energy density is low but power density is high, the battery can power a lot of appliances but for a very short time.

What is the Energy Density of LiFePO4 Batteries?

The energy density of a LiFePO4 estimates the amount of energy a particular-sized battery will store. Lithium-ion batteries are well-known for offering a higher energy density. Generally, lithium-ion batteries come with an energy density of 364 to 378 Wh/L. Lithium Iron Phosphate batteries lag behind in energy density by a small margin.

A higher energy density means a battery will store more energy for any given size. However, higher energy density is not always better. There are some advantages of the marginally lower energy density that LFP batteries have.

Higher energy density can make the battery potentially unsafe due to the amount of energy stored in it. This extra energy can come off as fire or explosions in case of short circuits or overcharging. Additionally, it also increases the possibility of a thermal runaway. However, LFP batteries avoid such occurrences.

Additionally, LFP batteries can provide higher energy than lithium-ion batteries by slightly increasing the dimensions. This is how LFP batteries outperform their lithium-ion counterparts in every aspect.

Is There a Limit to the Energy Density

The theoretical limit of any lithium-ion battery is 644 Wh/L. However, Li-ion batteries have only been developed to the 350-364 Wh/L ceiling. Even the high-capacity lithium-ion batteries of Tesla have an energy density of 250 350 Wh/L.

Many challenges come with storing a higher amount of energy in a battery. The high energy poses a major risk to life and property near the battery. Such high energy density Li-ion batteries will pack a lot of power even in small cells. Therefore, any damage to a cell due to surges or physical effects can cause explosive consequences and thermal runaway.

Importance of Battery Energy Density

It is important to consider the energy density when choosing a new battery. Higher energy density Li-ion batteries provide benefits like:

• Lightweight: Weight consideration is the primary benefit of a higher energy density. Battery technology with a higher density can be made lighter in size. This keeps the weight of the battery manageable while providing sufficient power output.
• Operating Time: Higher energy density means that a cell of given size can provide power for a longer time
• Power Output: A higher energy density means a higher overall power output. Therefore, you can power more appliances with higher wattage using LFP batteries.
• Cost Effective: High energy density cells require a low amount of materials to build a particular capacity energy solution. This makes the overall battery system cost-effective.
• Environmental Impact: Lesser resources are required to build a battery of a smaller size. This reduces the harmful effects of any battery on the environment. It also reduces the wastage caused when replacing Li-ion batteries.

What Factors Affect LiFePO4 Battery Energy Density?

The energy density of LiFePO4 batteries is not a prefixed value. Energy density is dependent on many factors, such as:

Cell Shape: The battery cell shape determines its energy density. Cylindrical cells provide the best energy density. Polymer cells, on the other hand, have a reduced energy density. The energy density of prismatic cells lies somewhere in between the two.

Manufacturing Quality: Good quality lithium cells have a uniform distribution of material. There are no air bubbles or contaminants. This ensures you are getting the maximum energy density. Poor-quality cells have defects that can reduce energy density significantly.

Temperature: Very cold temperatures halt the chemical reactions inside the battery. The energy density in these situations decreases significantly. Higher temperatures accelerate the reactions and increase the stored power. This increases the stored energy density. However, very high temperatures are detrimental to battery life and health.

Calculation of LiFePO4 Energy Density

Calculating the specific energy density of an LFP battery is quite easy. For these calculations, you require the Amp-hour capacity of the battery, its nominal voltage, and its mass.

Specific Energy Density = Voltage x Capacity ÷ Mass

The nominal voltage is the average voltage of the battery during one discharge cycle. It usually comes at around 3.2 V to 3.3 V.

The mass of the battery should include the weight of all battery components like electrodes, casings, plates, etc.

For instance, if you have a 200Ah LFP battery cell of weight 10 kg, the specific energy density will be:

• Specific Energy Density = 3.3 x 200 ÷ 10 = 66 Wh/kg

How to Optimise the Energy Density of a LiFePO4 Battery?

The key to increasing the energy density of a battery is to optimise its cathode (negative electrode) and anode (positive electrode). For the anode, you will have to improve the discharge characteristics of the battery. These are its discharge current and discharge capacity. You will have to improve the lithium ions removal rate for the cathode.

Endnotes

LFP batteries have a balanced energy density profile compared to other types of batteries. The significantly higher energy density than lead acid batteries ensures an exponential increase in capacity. The slightly lower energy density than conventional lithium batteries ensures safer storage and operation due to better chemical stability.

If you are wondering which battery to choose, LiFePO4 batteries can be worth your money and more. Check out the Eco Tree Lithium catalogue to learn about the various battery options that can fit your requirements.