In the battery industry, charge/discharge rates are commonly used to describe the relationship between charging speed and current. For example, a rate that fully charges the battery in one hour is called 1C, a rate that takes only 30 minutes is called 2C, and so on. Anything above 1C is considered fast charging. Currently, Lifepo4 batteries generally achieve charging rates of 1C-3C, with a maximum of around 5C. However, this is still far behind the discharge rates that often reach 10C.
Besides the bottleneck in maximum charging rate, the charging rate that the Lifepo4 battery can withstand varies at different SOC (State of Charge, i.e., remaining charge). Generally, the charging rate follows a slow-fast-slow pattern. Typically, when the SOC reaches above 90%, the internal resistance of the Lifepo4 battery increases significantly, slowing down the charging rate.
Therefore, if you are a user of a portable power station and want to save charging time as much as possible, try not to let the battery drain below 10% frequently. When charging, it doesn’t necessarily need to be fully charged; reaching 90% or higher, or enough range for your next trip, is sufficient. Besides the limitations of the Lifepo4 battery itself, external charging equipment also has its own limitations.
Theoretically, increasing the current can indeed increase charging speed. However, if the current is too high, the diffusion rate of lithium ions inside the Lifepo4 battery cannot keep up with the diffusion rate of electrons, leading to electron-ion disconnection, affecting battery performance, reducing the achievable charging capacity, and severely shortening the lifespan of the Lifepo4 battery. It may even pose a risk of fire or explosion.
Therefore, generally speaking, when not in a hurry, we recommend using slow charging as much as possible. This helps extend the lifespan of lithium iron phosphate batteries, and slow charging is safer.
During charging, the diffusion rate of lithium ions inside the lithium iron phosphate battery is closely related to temperature, cathode material, and structure.
First, temperature is crucial. Generally, higher temperatures result in faster diffusion, but excessively high temperatures can also lead to reduced battery life and decreased charging safety. Similarly, excessively low temperatures can cause lithium metal deposition in the battery, leading to internal short circuits, especially in lithium iron phosphate (LFP) batteries. Typically, at 0°C, LFP batteries retain only about 60-70% of their capacity, and at -20°C, this drops to a paltry 20-40%. Therefore, in cold northern winters, electric vehicles must have a heating function for the battery module, which naturally results in faster power consumption.
Second, materials are important. Different materials have vastly different diffusion capabilities. Lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, NCM, and NCA are all high-performance cathode materials, with the latter two being currently the best performing and most widely used. This is a major reason why LFP batteries are named after their cathode materials.
Basic Working Principle and Structure of Lithium Iron Phosphate Batteries
- Basic Battery Principle: The positive electrode undergoes a reduction reaction, gaining electrons; the negative electrode undergoes an oxidation reaction, losing electrons. Electrons pass through the load and flow from the negative electrode to the positive electrode, forming a current flowing from the positive electrode to the negative electrode.
- 1 (+1) valence lithium ion <—— (1-x) (+1/(1-x)) valence lithium ions + x (+1) valence lithium ions + x electrons
- Let x = 0.5, we get:
- 1 (+1) valence lithium ion <——0.5 (+2) valence lithium ions + 0.5 (+1) valence lithium ions + 0.5 electrons
- Multiply both sides by 2, we get:
- 2 (+1) valence lithium ions <——1 (+2) valence lithium ion + 1 (+1) valence lithium ion + 1 electron
Further simplification:
- 1 (+1) valence lithium ion <——1 (+2) valence lithium ion + 1 electron
This formula actually describes the overall reaction, not the reaction of a single individual cell. In simple terms:
- The positive electrode has a (+1/(1-x)) valence (where 0 is zero).
The lithium atoms at the negative electrode lose electrons and are oxidized to (+1) valence lithium ions. Electrons flow from the negative electrode into the load circuit; lithium iron phosphate ions flow to the positive electrode through the electrolyte.
This brings us back to the basic principle of the Lifepo4 battery. The core of the positive electrode is the (+1/(1-x)) valence lithium ion, and the core of the negative electrode is the lithium atom. The two react to generate (+1) valence lithium atoms, and the electron flow in the redox reaction forms the current.
In reality, when making batteries, a material is always needed to carry the lithium ions at the positive electrode and the lithium atoms at the negative electrode, just like goods always need shelves. The shelf for lithium ions is cobalt oxide. Lithium ions, together with lithium ions, form the positive electrode; the negative electrode’s lithium atoms are composed of porous graphite and other materials to prevent the negative electrode from being destroyed during the reaction. Between the positive and negative electrodes are the electrolyte and a separator, which both facilitate lithium ion flow and isolate the positive and negative electrodes to prevent internal short circuits.
Why discuss the basic working principle and structure of lithium iron phosphate batteries? This will be used later when discussing the charging and discharging cutoff voltages of Lifepo4 batteries and the dangers of overcharging and over-discharging.
Why is there a charging cutoff voltage?
In other words, what problems occur after overcharging? As mentioned earlier when describing the lithium-ion battery structure, the negative electrode is composed of graphite and lithium atoms. However, lithium does not exist in atomic form but rather as lithium ions coexisting with graphite. After overcharging, lithium ions will precipitate as crystalline lithium, unable to participate in charging and discharging, leading to a reduction in battery capacity.
Why does this happen?
There’s a discharge cutoff voltage. In other words, what problems arise from over-discharge? Over-discharge causes a large number of lithium ions to flow from the negative electrode to the positive electrode, leading to vacancy in the graphite and collapse in some areas, making it unable to store lithium ions and reducing battery capacity.
So what exactly limits the charging speed of lithium iron phosphate batteries?
Ultimately, it comes down to materials and technology, as mentioned earlier. Lithium iron phosphate doesn’t exist in atomic form; it needs to coexist with graphite. The process of fully charging a lithium iron phosphate battery involves lithium ions moving between the positive and negative electrodes, carrying and releasing ions to perform its electrical storage function. This requires a certain reaction time. Charging too quickly can cause abnormal reactions in the lithium battery, leading to crystallization. Furthermore, if the charging speed exceeds the battery’s tolerance, the internal resistance of the lithium battery will increase, causing the battery to overheat and become dangerous.
