In the actual use of lithium-ion battery, in addition to 3C class electronic devices, very few batteries are used alone, usually through series or parallel methods such as the formation of battery pack, under the control of the BMS external circuit power supply. In the lithium-ion battery composition The purpose of this purpose is to improve the consistency of lithium-ion batteries so as to avoid over-charging or over-discharging of some of the batteries in the battery pack, so as to improve the life of the battery pack. In general, the batteries are connected in series , You need to prioritize matching capacity to avoid overcharge or overdrain of a series connected battery during discharging.When the battery is connected in parallel, you need to match the internal resistance and capacity of the battery preferentially to reduce the battery's internal resistance during discharge Different current distribution caused by uneven, to avoid over-discharge of some batteries or overcharge.
For the parallel battery, due to the existence of a self-balancing process between the parallel cells, that is, during the discharge process, some cells with different internal resistance may experience overcharge or over discharge problems. However, Voltage difference can drive parallel battery rebalance, so to some extent, can reduce the impact of different resistance.But when the battery in the high current discharge process, due to the current is too large, may be in this self-balancing Before the action, some of the batteries were seriously overcharged or overdischarged, which led to the battery life is greatly affected.Recently, the United States Navy Laboratory LRL parallel LFP batteries in high-current pulse discharge life decline Rachel Carter et al. Found that there is no significant change in the positive electrode of LFP after cycling, and the negative electrode is the main factor leading to the decay of battery capacity. In the parallel state, high current pulse discharge easily lead to Part of the battery over-discharge, leading to dissolution of the negative electrode copper foil, and in the negative electrode and the re-deposition occurred on the diaphragm, change the copper foil Interface characteristics of the active material, the negative electrode active material causes the occurrence of spalling and delamination, leading to a parallel decline down LFP battery life accelerated.
In the experiment, Rachel Carter uses a 2.6Ah LFP cell, the positive electrode material is LiFePO4, and the negative electrode is graphite material.The cell works in two ways: the first one is a single cell for charge and discharge, the second one is after screening and matching Of the four batteries in parallel charge and discharge, the battery system as shown below.
1) 10C constant current discharge 4 seconds, let stand for 2 seconds, and then 10C constant current discharge 4 seconds, and then allowed to stand for 2 seconds, so repeated 50 times, and then the battery according to the 1C constant rate discharge to 2.0V.
2) 1C constant current charge to 3.5V, then constant voltage charge to 3.6V.
3) Repeat 1, 2 two steps 25 times.
4) The battery will be completed 25 test cycles in accordance with 0.5C charge and discharge system for the remaining capacity test.
5) If the remaining capacity> 80%, then the battery re-start from the first step test.
The test found that LFP cells, working alone, had a 28% decline in battery capacity after 1200 cycles, while the parallel connected cells reached 35% capacity decay after only 750 cycles. At the end of their life, after completing the pulsed discharge Of the rest time, the battery within the re-equilibrium current reached 1A, which shows that some of the batteries in the battery for over-discharge, and studies have shown that this rebalancing current is often caused by parallel lithium-ion battery pack life decline accelerated The important reason (the current mechanism is not yet clear).
To investigate the mechanism of battery life degradation described above, Rachel Carter will first conduct a capacity test on fully discharged batteries using CT. The LFPs were studied in three levels at a total of 1) battery level, with high energy (120 keV ) For a long period of time (3h) to obtain the image of the cell structure so as to check the quality of the cell and whether there is a macroscopic defect. 2) Electrode layer, RachelCarter removed some of the electrodes (0.5 * 1cm) (CT) was used to scan the electrodes for high energy (80KeV) and long time (3h) to analyze the electrode composition and the copper foil damage. 3) For the particle level, the electrode was scanned for a longer time (20h) For a higher resolution (218 nm) image, at this resolution we can analyze the interaction between the active material and the current collector.
The picture above shows a 'battery level' CT scan of the image showing a bright color in the image due to the higher density of the copper foil and the stainless steel housing, while the Al foil and graphite material appear black due to the lower density, So that we can distinguish the internal structure of the battery.
LFP battery is divided into four segments, there are three ears, the positive by 5 segments, with four ears, the positive and negative pole position in the chart c made a logo, which is represented by the white Negative pole position, the black semi-circle represents the location of the positive pole. From the scan results we can observe some production defects, as can be seen from the CT images (Figure d and Figure e), the copper foil appeared on the part However, Rachel Carter considers this to be a more common manufacturing defect and does not have a decisive effect on the cycle life of LFP cells.
To further investigate the decay mechanism of LFP cells, Rachel Carter dissected the circulated LFP cells and the anatomical cell images are shown below.From the anatomical cell view, three cells (no circulation, single Battery cycle and parallel post-cycle) have no obvious defects.After a single battery cycle of 1200 times, the negative electrode on both sides of the edge of the location of the apparent negative stratification, the middle of the negative electrode showed a light orange color , Which means that part of the active material in the negative electrode is in Stage 1 state of lithium insertion, indicating that some of the lithium-intercalated negative actives are deactivated in the cycle, while the negative electrode surfaces of the negative electrode exposed in parallel are bright blue and The purple color of the surface of the separator showed that obvious copper deposition occurred on the negative electrode and the separator. The XPS study showed that this part of copper is in the state of metal Cu.
From the above analysis, we can easily see that, whether it is a single battery cycle LFP battery, or parallel LFP battery after a long cycle, LFP cathode material did not occur significant changes in properties, indicating that the LFP material is good Stability. The change of negative polarity is the main factor leading to the decline of cycle life of LFP battery. In the single cycle of LFP battery, obvious delamination and exfoliation of active material appear on both sides of the negative electrode, while the middle position shows a shallow Orange, indicating that some lithium intercalation state of graphite active material deactivation occurred in the cycle.And the parallel cycle of LFP cells on the surface of the anode and the surface of the separator observed significant Cu deposition, indicating that during the cycle of Cu foil may A more serious dissolution problem has occurred, so dissolution of Cu foil is a major contributor to the decay of LFP capacity in parallel cycles.In the next article we will describe how to use CT tools in parallel with teams of active material particles in parallel Capacity decline to accelerate the phenomenon of analysis, so stay tuned.
