Lithium iron phosphate (LFP) batteries are lithium-ion batteries that use lithium iron phosphate as the positive electrode material. The main positive electrode materials for lithium-ion batteries include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, ternary materials, and lithium iron phosphate.
The cathode material for lithium-ion batteries is lithium iron phosphate, which offers significant advantages in safety and cycle life—key technical indicators for power batteries. It can achieve 2000 charge-discharge cycles at 1C, is puncture-resistant without exploding, and is less prone to combustion and explosion during overcharging. Lithium iron phosphate cathode materials also facilitate the parallel and series connection of high-capacity lithium-ion batteries.
Lithium iron phosphate (LFP) batteries are lithium-ion batteries that use lithium iron phosphate as the positive electrode material. The main positive electrode materials for lithium-ion batteries include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, ternary materials, and lithium iron phosphate. Among these, lithium cobalt oxide is currently the most widely used positive electrode material in lithium-ion batteries. In terms of material principles, lithium iron phosphate also involves an intercalation and deintercalation process, a principle identical to that of lithium cobalt oxide and lithium manganese oxide.
1. Introduction
Lithium iron phosphate batteries are lithium-ion rechargeable batteries, and one of their main applications is in power batteries. They have significant advantages over Ni-MH and Ni-Cd batteries.
Lithium iron phosphate batteries have high charge and discharge efficiency, reaching over 90% under high-rate discharge conditions, while lead-acid batteries have an efficiency of approximately 80%.
2. Eight major advantages
Improved safety performance
The PO bonds in lithium iron phosphate crystals are stable and difficult to decompose. Even at high temperatures or under overcharge, they do not collapse and generate heat or form strong oxidizing substances like lithium cobalt oxide, thus exhibiting good safety. Reports indicate that in actual operation, a small number of samples showed combustion in nail penetration or short-circuit tests, but no explosions were observed. However, in overcharge tests using voltages several times higher than their discharge voltage, explosions were still observed. Nevertheless, its overcharge safety is significantly improved compared to ordinary liquid electrolyte lithium cobalt oxide batteries.
Improved lifespan
Lithium iron phosphate batteries are lithium-ion batteries that use lithium iron phosphate as the positive electrode material.
Long-life lead-acid batteries have a cycle life of around 300 cycles, with a maximum of 500 cycles. Lithium iron phosphate (LFP) batteries, on the other hand, have a cycle life exceeding 2000 cycles, reaching 2000 cycles under standard charging (5-hour rate). Lead-acid batteries of the same quality typically last only 1-1.5 years – "new for six months, old for six months, and then another six months of maintenance" – while LFP batteries, under the same conditions, theoretically have a lifespan of 7-8 years. Considering all factors, the performance-price ratio is theoretically more than four times that of lead-acid batteries. LFP batteries also offer high-current discharge and fast charging/discharging capabilities, allowing for 2C fast charging and discharging. With a dedicated charger, a 1.5C charge can fully charge the battery within 40 minutes, and the starting current can reach 2C – capabilities not found in lead-acid batteries.
Good high temperature performance
Lithium iron phosphate (LFP) has a peak thermal conductivity of 350℃-500℃, while lithium manganese oxide and lithium cobalt oxide are only around 200℃. It has a wide operating temperature range (-20℃ to +75℃) and high-temperature resistance.
Large capacity
It has a larger capacity than ordinary batteries (such as lead-acid batteries). The capacity of a single cell ranges from 5AH to 1000AH.
Memoryless effect
When rechargeable batteries are frequently left fully charged without being completely discharged, their capacity will rapidly drop below the rated value; this phenomenon is called the memory effect. Nickel-metal hydride and nickel-cadmium batteries exhibit this memory effect, while lithium iron phosphate batteries do not. These batteries can be charged and used immediately regardless of their state, without needing to be fully discharged before recharging.
Lightweight
A lithium iron phosphate battery of the same capacity is 2/3 the volume of a lead-acid battery and 1/3 the weight of a lead-acid battery.
Environmental friendly
This battery is generally considered to be free of any heavy metals and rare metals (nickel-metal hydride batteries require rare metals), non-toxic (SGS certified), pollution-free, and compliant with European RoHS regulations, making it an absolutely green and environmentally friendly battery. Therefore, the main reason lithium batteries are favored by the industry is their environmental benefits. Consequently, this battery was included in the "863" National High-Tech Development Program during the "15th Five-Year Plan" period, becoming a key project supported and encouraged by the state. With China's accession to the WTO, the export volume of Chinese electric bicycles will increase rapidly, and electric bicycles entering Europe and the United States are now required to be equipped with pollution-free batteries.
However, experts say that the environmental pollution caused by lead-acid batteries mainly occurs in the unregulated production process and recycling stages. Similarly, while lithium batteries belong to the new energy industry, they are not immune to heavy metal pollution. During metal processing, substances such as lead, arsenic, cadmium, mercury, and chromium can be released into dust and water. Batteries themselves are chemical substances, so they can potentially generate two types of pollution: first, pollution from process waste during production; and second, pollution from discarded batteries.
Lithium iron phosphate batteries also have their drawbacks: for example, poor low-temperature performance, low tap density of the cathode material, and a larger volume for a lithium iron phosphate battery of the same capacity compared to lithium-ion batteries such as lithium cobalt oxide, thus lacking advantages in micro-batteries. When used in power batteries, lithium iron phosphate batteries, like other batteries, face the challenge of battery consistency.
Comparison of power batteries
Currently, the most promising cathode materials for power lithium-ion batteries are modified lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and lithium nickel cobalt manganese oxide (Li(Ni,Co,Mn)O2) ternary materials. Due to the scarcity of cobalt resources and the high cost and price volatility of nickel and cobalt, lithium nickel cobalt manganese oxide ternary materials are generally considered unlikely to become the mainstream for power lithium-ion batteries in electric vehicles, but they can be used in combination with spinel lithium manganese oxide within certain limits.
Industry Applications
Carbon-coated aluminum foil brings technological innovation and industrial upgrading to the lithium battery industry; it improves the performance of lithium battery products and enhances the discharge rate.
As domestic battery manufacturers increasingly demand higher battery performance, the following materials are widely recognized in China for new energy batteries: conductive materials, conductive coated aluminum foil, and copper foil.
Its advantages are: when processing battery materials, it often has good high-rate charge and discharge performance and large specific capacity, but poor cycle stability and serious degradation, so it has to make trade-offs.
This is a magical coating that enhances battery performance and ushers in a new era.
The conductive coating is composed of well-dispersed nano-conductive graphite-coated particles. It provides excellent static conductivity and acts as a protective energy absorption layer. It also offers good shielding properties. The coating is available in water-based and solvent-based formulations and can be applied to aluminum, copper, stainless steel, and aluminum and titanium bipolar plates.
Carbon coating improves the performance of lithium batteries in the following ways:
1. Reduce battery internal resistance and suppress the increase in dynamic internal resistance during charge-discharge cycles;
2. Significantly improves battery pack consistency and reduces battery pack costs;
3. Improve the adhesion between the active material and the current collector, and reduce the manufacturing cost of the electrode;
4. Reduce polarization, improve rate performance, and reduce thermal effects;
5. Prevent the electrolyte from corroding the current collector;
6. These combined factors extend battery life.
7. Coating thickness: 1-3 μm on one side (standard).
In recent years, Japan and South Korea have mainly developed power lithium-ion batteries using modified lithium manganese oxide and nickel-cobalt-manganese lithium ternary materials as cathode materials. Examples include Panasonic EV Energy Co., Ltd., a joint venture between Toyota and Panasonic, as well as Hitachi, Sony, Shin-Kobe Electric, NEC, Sanyo Electric, Samsung, and LG.
The United States primarily develops power lithium-ion batteries using lithium iron phosphate as the cathode material, such as those from A123 Systems and Valence. However, major US automakers have opted for manganese-based cathode materials in their PHEVs and EVs. Furthermore, it's rumored that A123 Systems is considering entering the lithium manganese oxide material field. Meanwhile, European countries like Germany mainly develop electric vehicles through collaborations with battery companies from other countries, such as the Daimler-Benz alliance with Saft and the Volkswagen-Sanyo agreement. Currently, Volkswagen in Germany and Renault in France are also researching and producing power lithium-ion batteries with government support.
shortcoming
Whether a material has potential for application and development depends not only on its advantages, but more importantly on whether it has fundamental defects.
In China, lithium iron phosphate is now widely chosen as the cathode material for power lithium-ion batteries. Market analysts from the government, research institutions, enterprises, and even securities companies are optimistic about this material and regard it as the development direction for power lithium-ion batteries.
The main reasons for this are as follows: First, it was influenced by the research and development direction in the United States, with companies like Valence and A123 being the first to use lithium iron phosphate as the cathode material for lithium-ion batteries. Second, China has not yet produced lithium manganese oxide materials with good high-temperature cycling and storage performance suitable for power lithium-ion batteries. However, lithium iron phosphate also has fundamental drawbacks that cannot be ignored, which can be summarized as follows:
1. During the sintering process of lithium iron phosphate preparation, iron oxide may be reduced to elemental iron under a high-temperature reducing atmosphere. Elemental iron can cause micro-short circuits in the battery, making it a highly undesirable substance for batteries. This is the main reason why Japan has consistently avoided using this material as the cathode material for power lithium-ion batteries.
2. Lithium iron phosphate (LFP) batteries have some performance defects, such as very low tap and compaction densities, resulting in low energy density. Their low-temperature performance is also poor, a problem that has not been solved even with nano-sizing and carbon coating. Dr. Don Hillebrand, director of the Energy Storage Systems Center at Argonne National Laboratory, described the low-temperature performance of LFP batteries as "terrible," stating that their tests showed LFP batteries cannot power electric vehicles at temperatures below 0°C. Although some manufacturers claim that LFP batteries retain good capacity at low temperatures, this is under conditions of low discharge current and very low discharge cutoff voltage. Under these conditions, the equipment simply cannot start or operate.
3. The high cost of material preparation and battery manufacturing results in low battery yield and poor consistency. While nano-sizing and carbon coating of lithium iron phosphate improve the electrochemical performance of the material, they also bring other problems, such as reduced energy density, increased synthesis costs, poor electrode processing performance, and stringent environmental requirements. Although lithium iron phosphate is rich in the chemical elements Li, Fe, and P, and their cost is relatively low, the cost of the prepared lithium iron phosphate products is not low. Even after deducting the initial R&D costs, the material processing cost plus the high cost of battery manufacturing results in a high cost per unit of energy storage.
4. Poor product consistency. Currently, no domestic lithium iron phosphate material manufacturer has been able to solve this problem. From a materials preparation perspective, the synthesis reaction of lithium iron phosphate is a complex multiphase reaction involving solid-phase phosphate, iron oxides and lithium salts, an added carbon precursor, and a reducing gas phase. In this complex reaction process, it is difficult to ensure the consistency of the reaction.
5. Intellectual Property Issues. The earliest patent application related to lithium iron phosphate was granted by FXMITTERMAIER & SOEHNEOHG (DE) on June 25, 1993, with the results published on August 19 of the same year. The fundamental patent for lithium iron phosphate is owned by the University of Texas, while the carbon coating patent was filed by a Canadian. These two fundamental patents are unavoidable; if patent royalties are factored into the cost, the product cost will increase further.
Furthermore, in terms of experience in the research and development and production of lithium-ion batteries, Japan was the first country to commercialize lithium-ion batteries and has consistently dominated the high-end lithium-ion battery market. While the United States leads in some basic research, it currently lacks a large-scale lithium-ion battery manufacturer. Therefore, Japan's choice of lithium manganese oxide as the cathode material for power lithium-ion batteries makes more sense. Even in the United States, manufacturers using lithium iron phosphate and lithium manganese oxide as cathode materials for power lithium-ion batteries are roughly equal, and the federal government supports the research and development of both systems simultaneously.
Given the aforementioned problems with lithium iron phosphate, it is difficult for it to be widely used as a cathode material in power lithium-ion batteries, particularly in fields such as new energy vehicles. However, if the problems of poor high-temperature cycling and storage performance of lithium manganese oxide can be solved, its advantages of low cost and high rate performance will make it a promising candidate for application in power lithium-ion batteries.
6. Working Principle and Characteristics The full name of a lithium iron phosphate battery is lithium iron phosphate lithium-ion battery, but this name is too long, so it is simply called lithium iron phosphate battery. Because its performance is particularly suitable for power applications, the word "power" is added to the name, i.e., lithium iron phosphate power battery. Some people also call it "lithium iron (LiFe) power battery".
significance
In the metals trading market, cobalt (Co) is the most expensive and has limited reserves, while nickel (Ni) and manganese (Mn) are cheaper, and iron (Fe) is the cheapest. The price of cathode materials also follows the price trends of these metals. Therefore, lithium-ion batteries made with LiFePO4 cathode material should be the cheapest. Another advantage is that it is environmentally friendly.
The requirements for rechargeable batteries are: high capacity, high output voltage, good charge/discharge cycle performance, stable output voltage, ability to charge and discharge at high currents, electrochemical stability, safety during use (not prone to combustion or explosion due to overcharging, over-discharging, or short circuits), wide operating temperature range, non-toxic or low-toxicity, and no environmental pollution. Lithium iron phosphate batteries using LiFePO4 as the positive electrode perform well in all these aspects, especially in high discharge rate (5-10C discharge), stable discharge voltage, safety (no combustion or explosion), lifespan (number of cycles), and no environmental pollution. They are currently the best high-current output power batteries available.
Structure and working principle
The internal structure of the LiFePO4 battery consists of olivine-structured LiFePO4 as the positive electrode, connected to the battery by aluminum foil. A polymer separator separates the positive and negative electrodes, allowing lithium ions (Li+) to pass through while blocking electrons (e-). The negative electrode, composed of carbon (graphite), is connected to the battery by copper foil. The electrolyte is located between the top and bottom of the battery, which is then sealed in a metal casing.
In a LiFePO4 battery, during charging, lithium ions (Li+) migrate from the positive electrode to the negative electrode through the polymer separator; during discharging, lithium ions (Li+) migrate from the negative electrode to the positive electrode through the separator. The lithium-ion battery is named for this back-and-forth migration of lithium ions during charging and discharging.
Main performance
The nominal voltage of a LiFePO4 battery is 3.2V, the termination charging voltage is 3.6V, and the termination discharging voltage is 2.0V. Due to differences in the quality and manufacturing processes of the positive and negative electrode materials and electrolyte materials used by different manufacturers, their performance may vary. For example, the capacity of the same model (standard battery with the same package) can differ significantly (10%–20%).
It should be noted that lithium iron phosphate power batteries produced by different factories may have some differences in various performance parameters; in addition, some battery performance parameters are not listed, such as battery internal resistance, self-discharge rate, and charge/discharge temperature.
Lithium iron phosphate (LFP) batteries vary considerably in capacity and can be categorized into three types: small (a few tenths of an ampere-hour to a few milliampere-hours), medium (tens of milliampere-hours), and large (hundreds of milliampere-hours). The same parameters also differ slightly between different types of batteries. Currently, the most widely used small, standard cylindrical LFP battery has the following dimensions: diameter 18mm, height 650mm (model 18650).
Over-discharge to zero voltage test
A discharge-to-zero voltage test was conducted using an STL18650 (1100mAh) lithium iron phosphate power battery. Test conditions: The 1100mAh STL18650 battery was fully charged at a 0.5C charging rate, then discharged at a 1.0C discharging rate until the battery voltage reached 0V. The batteries were then divided into two groups: one group was stored for 7 days, and the other for 30 days. After the storage period, they were fully charged again at a 0.5C charging rate, and then discharged at a 1.0C discharging rate. Finally, the differences between the two zero-voltage storage periods were compared.
The results of the test were that after 7 days of zero-voltage storage, the battery showed no leakage, good performance, and 100% capacity; after 30 days of storage, it showed no leakage, good performance, and 98% capacity; after 30 days of storage, the battery was subjected to 3 charge-discharge cycles and its capacity recovered to 100%.
This test demonstrates that even if the battery is over-discharged (even to 0V) and stored for a certain period, it will not leak or be damaged. This is a characteristic not found in other types of lithium-ion batteries.
Features of lithium iron phosphate batteries
Based on the above introduction, the characteristics of LiFePO4 batteries can be summarized as follows.
High-efficiency output: Standard discharge is 2-5C, continuous high-current discharge can reach 10C, and instantaneous pulse discharge (10S) can reach 20C;
It performs well at high temperatures: the internal temperature reaches as high as 95°C when the external temperature is 65°C, and the temperature can reach 160°C when the battery is fully discharged, while the battery structure remains safe and intact;
Even if the battery is damaged internally or externally, it will not burn or explode, offering the best safety; it has excellent cycle life, retaining more than 95% of its discharge capacity after 500 cycles;
It is undamaged even when over-discharged to zero volts; it can be charged quickly; it is low-cost; and it does not pollute the environment.
Application of lithium iron phosphate power batteries
Because lithium iron phosphate batteries possess the aforementioned characteristics and are produced in various capacities, they have quickly gained widespread application. Their main application areas include:
Large electric vehicles: buses, electric cars, sightseeing vehicles, and hybrid vehicles, etc.;
Light electric vehicles: electric bicycles, golf carts, small flatbed electric carts, forklifts, cleaning vehicles, electric wheelchairs, etc.;
Power tools: electric drills, chainsaws, lawnmowers, etc.;
Remote control cars, boats, airplanes, and other toys;
Energy storage devices for solar and wind power generation;
UPS, emergency lights, warning lights, and mining lamps (for the highest safety);
Replaces the 3V disposable lithium battery and the 9V nickel-cadmium or nickel-metal hydride rechargeable battery in cameras (identical in size);
Small medical instruments and portable devices, etc.
Here's an example of using lithium iron phosphate (LFP) batteries to replace lead-acid batteries. A 36V/10Ah (360Wh) lead-acid battery weighs 12kg, can travel approximately 50km on a single charge, has about 100 charge cycles, and a lifespan of about one year. A LFP battery, using the same 360Wh capacity (composed of 12 10Ah batteries connected in series), weighs about 4kg, can travel approximately 80km on a single charge, has up to 1000 charge cycles, and a lifespan of 3-5 years. Although LFP batteries are significantly more expensive than lead-acid batteries, the overall economic benefit is better, and they are also more convenient to use.
Battery performance
The performance of lithium-ion power batteries primarily depends on the positive and negative electrode materials. Lithium iron phosphate (LFP) is a relatively recent development in lithium battery technology, with China's first large-capacity LFP battery appearing in July 2005. Its safety performance and cycle life are unmatched by other materials, which are also the most important technical indicators for power batteries. It boasts a 1C charge-discharge cycle life of 2000 cycles. A single cell will not burn under a 30V overcharge voltage and will not explode upon puncture. LFP cathode materials facilitate the series connection of large-capacity lithium-ion batteries to meet the frequent charging and discharging needs of electric vehicles. With advantages such as being non-toxic, pollution-free, safe, having widely available raw materials, being inexpensive, and having a long lifespan, it is an ideal cathode material for next-generation lithium-ion batteries.
This project falls under the category of functional energy materials development within high-tech projects, and is a key area supported by the National "863" Program, "973" Program, and the "Eleventh Five-Year Plan" for the development of high-tech industries.
The cathode material for lithium-ion batteries is lithium iron phosphate, which offers significant advantages in safety and cycle life—key technical indicators for power batteries. It can achieve 2000 charge-discharge cycles at 1C, is puncture-resistant without exploding, and is less prone to combustion and explosion during overcharging. Lithium iron phosphate cathode materials also facilitate the parallel and series connection of high-capacity lithium-ion batteries.
Scientific Research Application
Lithium iron phosphate batteries
Recently, there have been continuous reports about the progress of new batteries that are expected to replace traditional lithium batteries, giving us hope that mobile phones and tablets will have longer battery life. However, unfortunately, most of them are still in the laboratory research stage, and it is hard to say when or even if they can be put into large-scale commercial use.
According to the lithium iron phosphate battery technology white paper published by DebochTEC GmbH, the energy density of a single 32650 cell (32mm in diameter/65mm in length) can be increased to 6000mAh after using composite nanomaterials. Compared with the current industry standard of 5000mAh per 32650 cell, the energy density is increased by 1000mAh, or 20%, for the same volume. One cell can charge an iPhone 4S approximately four times.
Even more encouraging is that, when used in a low-rate charge-discharge environment, this battery retains about 80% of its charge after up to 3,000 cycles, while ordinary lithium batteries only last about 500 cycles. Based on a charge-discharge cycle of once every 3 days, it can last for 24 years, making it a truly long-life battery.
This new battery technology can be widely used in various devices such as portable power banks, small UPS, laptop batteries, and car batteries. Moreover, DebochTEC.GmbH uses different cell colors according to the number of charge cycles for different usage environments: gold for special-grade batteries with 3,000 cycles; blue for civilian vehicles with 2,500 cycles; and green for small portable mobile devices with 2,000 cycles.
