Long-life cycle lithium iron phosphate batteries, materials and processes

Taking advantage of the lithium-rich advantages of prelithiation technology and prelithiation materials (also known as lithium replenishing agents and prelithiation additives), the prelithiation materials are added to the positive electrode sheets of lithium iron phosphate (LFP) batteries to develop phosphoric acid The lithium-iron square aluminum-cased 51 Ah full battery and the soft-pack 7 Ah full battery have been tested and researched on cycle life. Using past experience, we analyzed the characteristics and characteristics of main materials such as positive and negative electrodes (such as lithium iron phosphate, graphite, electrolyte ratio, current collector, separator) to auxiliary materials (positive and negative electrode binders, conductive agents, etc.) as well as pre-lithiation materials. For the impact on battery life, the lithium replenishment mechanism and lithium replenishment capacity value of several industrialized pre-lithium materials are analyzed in the form of chemical reaction equations. The test results show that the cycle life of the 7 Ah battery with pre-lithium material added is 9000 cycles, while the cycle life of the 7 Ah battery without pre-lithium material is 5300 weeks. The cycle life of the 7 Ah battery with pre-lithium material added is obvious. Better than batteries without pre-lithium materials, the cycle life is increased by about 50%; in terms of energy efficiency: the 7 Ah battery with pre-lithium materials has an energy conversion efficiency of 96.74% at room temperature of 25°C at 0.2 C and 96.74% at 0.5 C. The efficiency is 94.80%, and the 1 C energy conversion efficiency is 92.67%, both of which are higher than the energy conversion efficiency of the 7 Ah battery without adding pre-lithium materials. This research will help promote the application of prelithiation technology and prelithiation materials in new long-cycle lithium iron phosphate energy storage batteries, and provide experimental basis for the design and development of long-life lithium iron phosphate batteries.

The electrolyte ratio (including additives) is also one of the key technologies to improve the life of lithium iron phosphate batteries. Among them, the addition ratio and order of the film-forming additive vinylene carbonate (VC) have an important impact on the battery cycle life, such as primary and secondary The solution of sub-injection of VC electrolyte with different contents can reduce the internal resistance of the battery and increase the residual film-forming additive VC content of the electrolyte in the battery after composition. For example, use vinylene carbonate with a content of 1.0% to 2.0% (mass fraction). The battery is injected once with the ester electrolyte to obtain a semi-finished battery and preformed, and then the electrolyte containing 6.0% to 16.0% (mass fraction) vinylene carbonate is used for the second injection. The liquid volume is 5% to 15% of the total amount of liquid injected to obtain a lithium iron phosphate battery with a long cycle life; other film-forming additives such as vinyl sulfate, FEC, LiBOB, LiDFOB, etc. also have a good effect on improving the battery cycle life.

The selection of current collector is also one of the key technologies to improve the life of lithium iron phosphate batteries. The functional conductive coating performs surface treatment on the battery conductive substrate (current collector) to form carbon-coated aluminum foil, which is to disperse dispersed nano conductive graphite or carbon nanotubes, etc. The conductive agent slurry is evenly and finely coated on the aluminum foil. It can provide excellent static conductive properties and collect the microcurrent of active materials, thereby greatly reducing the contact resistance between the cathode material and the current collector, improving the adhesion between the two, and increasing the internal resistance of the battery. Consistency, thereby improving the overall performance of the battery such as cycle life. The higher the conductivity of the conductive coating of carbon-coated aluminum foil (30 S/cm) and the thinner the coating thickness (0.3 μm on one side), the better it is for the battery. Performance improvements.

Choosing a binder with strong peeling force and low rebound is beneficial to the long cycle life of the battery. The negative electrode binder sodium polyacrylate PAA has a sodium oxygen group and has a high peeling force with the foil and the electrode piece is highly compacted. It has the advantages of small density and rebound, and can obtain long cycle life performance, but the pole piece is relatively hard, which can be improved by adding a small amount of particle structure styrene-butadiene rubber (SBR).

A reasonable compaction density can improve the performance of the conductive agent, reduce the amount of conductive agent, and improve the liquid retention coefficient of the battery system. If the compaction density is low, the conductive agent will have poor conductive effect and the battery impedance will be large. Generally, the compaction density should be greater than 2.1 g/cm3. If the compaction density is too high, the active material particles are easily broken, and the active material particles are packed too closely together, which increases the electronic conductivity, but the ion transmission power becomes poor and the electrolyte infiltration effect is poor, which in turn affects the capacity development. Generally, the compaction density should be less than 2.5 g/cm3.

The selection of positive and negative electrode powder materials has a great impact on long cycle life. Generally, lithium iron phosphate cathode materials with small single crystal particles have better cycle life. At the same time, it is necessary to consider the blending of large and small particles, good carbon coating, and good conductivity. High-quality lithium iron phosphate cathode material; the anode material requires reasonable primary/secondary particle blending, taking into account material stability and dynamics, preferably needle coke/petroleum coke.