For the purpose of protecting the environment, the country is accelerating the replacement of a lithium-ion battery to replace the conventional lead-acid battery, using a lithium ion battery, replacing the lead-acid battery as a car to stop the power supply, the lithium-ion battery has very high magnification discharge capability, usually 20 -30C discharge rate, but we currently have a lack of systematic research on system design and life-limiting decline in high-magnification batteries.. Recently, the Alexanderschmidt of the German Karlsruhe Institute of Technology and Annasmith (Corresponding author), etc.
, the high-magnification discharge performance of different systems (LCO / LFP) batteries and the decline in large rate discharge (maximum 45C) cycle The reduction is analyzed and studied. The analysis shows that the magnification performance of the LCO / graphite system is better than the LFP / graphite system, and the mechanism of decline indicates that the capacity decline under high-magnification discharge is important to high-temperature discharge..
The batteries used in the experiment are shown in the following table. The positive material of most batteries is LCO, and some batteries are used by LFP. There are 8 battery structures in the 9 batteries.
A cylindrical structure (LFP2) The maximum magnification discharge capacity of the battery is as shown in the following table, and most batteries can be 50% (associated with 1C capacity) at the highest magnification, and only LCO2 battery magnification discharge capacity is poor, and the capacity retention rate at 45C It is only 7.7%, 65C is only 2.1%, and less than 0.
1% is less than 100c. The magnification discharge capacity is preferably LCO5 (65C capacity retention ratio 93.4%) and LCO7HV (65C capacity retention rate of 60.
1%). In the above-described nine cells, the soft bag lithium battery structure is Z-shaped laminate (as shown in Figure A), the cylindrical battery is 26650, the battery core is a winding structure, and the positive and negative electrode is respectively drawn from 4 poles ( As shown in Figure B below). The magnification negative electrode of several batteries can be divided into four categories: wherein the following figure A is a broken tabular graphite, most of the particle size is 1-5 um, and the portion large particle size can reach 10-15 um, graphite particle surface Smooth, battery LCO1 and LCO2 (from vendor a) use this type of graphite; the following figure B is a large-sized sheet-like graphite, and most of the particles can reach 5-10 um, and partially large particles can reach 20-25 um.
The surface of graphite particles is rough, battery LCO3, LCO4 and LFP1 (from vendor b) use this graphite. The following figure C is a third type of negative electrode, a negative electrode exhibits a spherical particles having a diameter of 10 um, a relatively rough surface, a battery LCO5, LCO6HV and LCO7HV (from vendor b) using this type of graphite. The following Down D shows the fourth graphite material, which has similar particle morphology similar to the second type of graphite, but the surface of the particles should be smoother, only the battery LFP2 is used.
. It can be seen from the following figure E and F, compared to spherical graphite, the sheet graphite can achieve a higher compaction density, and can see almost all negative electrodes from a larger magnification picture as a conductive agent. Only LFP2 batteries have also added gas phase growth carbon fiber (VGCF) in the negative electrode (VGCF).
The positive polar morphology is shown in the following figure, according to the histological LCO positive electrode can be divided into three categories: wherein the first category is shown in Figure A, the diameter of the LCO particles is about 1 um, and the part large particles can reach 2-3 um, LCO particles. Distributed a large number of conductive agent clusters (battery LCO1, LCO3, and LCO5 bits); the second class is shown in Figure B, which is similar to the first relatively similar, but the conductive agent is much less (battery LCO2 and LCO4 is this type); the third category is shown in Figure C, the LCO particle diameter is 5-6 um, which is important for reducing the particle ratio surface area, reducing the side reactions under high voltage (battery LCO6HV and LCO7HV for this type)..
The following figure D is the LFP positive electrode, exhibits a typical nano-LFP material.. Improve lithium-ion battery power density is the most effective way to reduce the thickness of the positive and negative electrode, such as the positive electrode and the negative thickness of the LCO3 and LCO6HV batteries of 42-55 um and 58-59um, respectively, respectively, and the thickness of the battery is 20-35um.
And 26-42um, so the power density of LCO3 and LCO6HV is only 1770-2000W / kg, far below other batteries (3700W / kg). Another method of increasing the power density is to increase the content of the conductive agent, such as the carbon content of LCO4 and LCO2 batteries of 3.2-3.
8%, its power density of 3700-4100 w / kg, while carbon content reaches 5.4-5.5% other batteries Power density can reach 5400W / kg or more (herein here is not only from conductive agents, but also carbon elements in binders).
Another major factor affecting the power density of the lithium-ion battery is the morphology of the active substance. The LCO7HV battery power density of the LCO particles is 5400 W / kg, and the power density of the LCO5 battery using small particles LCO can reach 7600 w / kg..
The following figure shows a circulating curve of 1C charging and different magnification discharges in several different systems. It can see that there is a significant gap between different batteries, using high-pressure LCO materials, LCO6HV and LCO7HV batteries The capacity loss in 10 cycles exceeded 10%, which may have a layer of passivation film on the surface of the positive electrode under high voltage..
Second, the discharge magnification has a significant effect on the decline of the battery, and the discharge ratio increases from 25c to 45C. The number of cycles will be reduced to the original 1/2 and 2/3, if the discharge magnification is reduced to 1C Then, the battery life will also increase significantly. Most batteries can more than 1,500, LCO1, LCO2, LCO7HV and LFP2 batteries after a circulatory capacity reach more than 90%, while LCO5 and LFP1 can reach more than 85%.
In order to analyze the cause of the lithium-ion battery recycle life after the high-magnification, the author will understand the battery after high-magnification discharge cycle, and the picture below is a SEM photo of several battery negatives, which can be seen from the figure. LFP2 batteries, the thickness of the inert layer after the negative electrode of several other batteries, especially for spherical negatives (LCO5, LCO6HV and LCO7HV battery negative electrodes), is particularly serious, and the topography of the original negative particles is almost Unrecognizable. The figure below shows the topography of the positive electrode, which can see the positive electrode in the figure, and the surface of the LCO particles also has an inert layer (the bright color portion in the figure), but compared to the thickness of the negative electrode inert layer.
More thin, and LFP positive is not significantly changed after the change in the cycle, indicating that the LFP material is good stability.. The following figure shows the AC impedance map of LCO2, LCO7HV, and LFP1 batteries in the loop.
From the figure, it can see that the EIS curve is important from the high frequency region inductance, the high frequency zone, the medium frequency zone, and the diffusion curve of the medium frequency zone semicircle and the low frequency zone. The equivalent circuit in the figure below is fitted, where R1 ohmic impedance, R2-bit SEI film impedance, R3 is charge exchange impedance, and changes in different batteries R1, R2, and R3 resistance after different magnification cycles, as shown in the table below, Seeing about the battery LCO1, LCO2, LCO4 and LCO5, R1 and R2 add more faster than the low rate cycle, which may be because of the temperature of these batteries in high magnification cycles, thus causing electrolyte The decomposition of the negative electrode is even more intense, resulting in a thickening of the SEI, so the R2 increases, and the electrolytic solution has decomposed a large amount of decomposition that causes ohmic impedance R1..
The battery LCO3 is very fast in low-magnification or high-rate.. The two types of battery using LFP positive electrodes are also very different.
Regarding the LFP1 battery, whether it is high and low rate, or the number of cycles is long, its R3 has added nearly ten times, and the LFP2 battery has almost no resistance in the cycle. Change, this shows that even if the battery of the battery of the same system is also different, it may not be the same..
From the perspective of the power density, the LCO / graphite system (7600W / kg) is better than the LFP / graphite system (2100W / kg), which is important because of the excellent electronic conductivity of the LCO material (especially in the high SOC state). Cause. In general, the design of high-magnification battery should follow three points: 1) Thin-thin electrodes; 2) More conductive agents; 3) Smaller particles.
Mechanism Analysis also indicates that the important cause of cyclic decline at high magnification is from high temperatures from the high-temperature discharge, and the electrolyte is decomposed on the surface of the electrolytic solution, causing the SEI film to thicken, so that the internal resistance of the battery New increase, may even result in lithium negative lithium during cycle, thereby accelerating the decay of lithium-ion batteries. .