Influence of overdischarge performance of lithium batteries with different preload forces

Influence of overdischarge performance of lithium batteries with different preload forces

1 Experiment

1.1 Electrochemical performance test

      The EIS of the battery was tested by electrochemical workstation, and the performance of the battery was tested by high-precision battery tester, with the range of 5V, 100~500A. Conventional capacity test method: charging to 3.65V with 1.00C constant current, then charging to 0.05C with constant voltage, setting aside for 0.5h, discharging to 2.50V with 1.00C, setting aside for 0.5h, and cycling for 3 times.

 

Battery cycle test method: charge to 3.65V with 1.00C constant current, turn to constant voltage charging to 0.05C cut-off, discharge to 2.50V with 1.00C, 2.00C, 5.00C respectively, repeat the cycle, shelve for 0.5h, until the cycle is 1000 times. Battery over-discharge test method: the base charge/discharge multiplier is 1.00C charging, 1.00C discharging, and the over-discharge voltages are 0~3.65V, 0.20~3.65V, 0.50~3.65V, 0.80~3.65V, and 1.50~3.65V, respectively. the conventional voltage is 2.50~3.65V. there is a fixture that indicates that the plane of the core is subjected to a preload of (1400±200)N. The preload of (1400±200)N is applied to the plane of the core. 200)N preload force; no fixture means 0N preload force is applied to the cell.

 

Battery shelf life test method: ① charging with 0.50C constant current to 3.65V, turn constant voltage charging to 0.05C cut-off, shelf 0.5h, discharge with 780W to 2.50V, record the capacity of 1; ② shelf 0.5h, charging with 0.50C constant current to 3.65V, turn constant voltage charging to 0.05C cut-off, shelf 0.5h, discharge with 780W to the specified voltage ( 1.50V, 0.80V, 1.50V, 0.80V, 0.80V). 1.50V, 0.80V, 0.50V), set aside for a specified time, charge to 3.65V with 0.10C constant current, turn to constant voltage charging to 0.05C cut-off, set aside for 0.5h, and discharge to 2.50V with 780W to record the discharged capacity 2; ③ Repeat step ①, record the discharged capacity 3, and repeat steps ①~③. Wherein, the ratio of capacity 2 to capacity 1 is the retention rate, and the ratio of capacity 3 to capacity 1 is the recovery rate.

2 Results and Discussion

2.1 Cycling performance at different multipliers The experiments were carried out on the basis of the battery used in the data center, and the multiplicity cycle performance of this battery is shown in Fig. 1. From Fig. 1, it can be seen that for the same multiplicity charging, the capacity decayed by 14% and 20% for 2000 cycles of 2.00C discharge and 5.00C discharge, which is 2 and 8 percentage points lower relative to the decay of 12% for 1.00C discharge cycle, respectively. In terms of cycling performance, the battery is characterized by multiplicative long cycling.

2.2 Performance characteristics of over-discharge cycles at different voltage ranges

In order to study the overdischarge characteristics of aluminum-cased batteries in different voltage intervals, the batteries were subjected to continuous cycling tests, with the variables being the voltage intervals and the state in which the batteries were placed during cycling: with clamps, without clamps. The batteries were numbered separately, and the content of the variations as well as the initial state of the batteries are listed in Table 1, with a state of charge (SOC) of 30%. From the initial voltage, internal resistance, and thickness of the 12 groups of batteries at 30% SOC listed in Table 1, the consistency of the batteries used for the experiment was good.



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The battery was subjected to continuous cycling test and the decay curve is shown in Fig. 2. From Fig. 2(a), it can be seen that the cycling performance of the battery gradually decayed with the deepening of the degree of overdischarge when the battery had no fixture. When the discharge cut-off voltage of the battery was 2.50V, 1.50V, 0.80V, 0.50V, 0.20V and 0V, the cycle times and capacity retention of the battery were 94.07% for 1000 cycles, 80.00% for 1000 cycles, 74.93% for 648 cycles, 67.72% for 360 cycles,63.26% for 119 cycles and 61.85% for 145 cycles, respectively 63.26% for 119 cycles and 61.85% for 145 cycles,respectively.

 



     When the lower limit voltage is reduced by 1.00V (from 2.50V to 1.50V), the decay rate increases by 14% for the same number of cycles; when the lower limit voltage is reduced by 1.70V (from 2.50V to 0.80V), the cycle life is only 648 cycles; and when the battery is discharged to 0V, the cycle life is even reduced to 145 cycles. The reduction of the lower limit voltage of discharge has a significant effect on the cycle life of the battery. Expanding the charging and discharging voltage range of the battery can increase the capacity of the battery, e.g., after the voltage is changed from 2.50~3.65V to 0~3.65V. The battery capacity is increased by 4Ah.

 

From Fig. 2(b), it can be seen that the battery exhibits the same performance law as without fixture even with fixture. The lower the discharge voltage of the battery, the shorter the cycle life. The number of cycles and retention rate of the battery at 2.50 V, 1.50 V, 0.80 V, 0.50 V, 0.20 V and 0 V discharge cutoff voltage were 96.03% for 1000 cycles, 86.73% for 928 cycles, 84.46% for 733 cycles, 81.95% for 741 cycles, 61.28% for 598 cycles and 60.08% for 647 cycles, in that order.

2.3 Performance characteristics of overdischarge cycle under different preloads

The deeper the overdischarge and the worse the cycle life of the battery with or without preload. The effect of the presence or absence of clamps on battery performance is further explored. By analyzing the data, the cycle performance of batteries with and without clamps is compared under the same working conditions.

 

Regardless of the actual cycle voltage range of the battery, the cycle performance of the battery with clamps is better than that without clamps. However, as the lower limit of the discharge voltage continues to rise, the effect of the fixture on the cycling performance of the battery gradually decreases. Between 0 and 3.65 V, the battery life with fixture was 647 times, while the battery life without fixture was 145 times, with a difference of 502 times; at 0.20-3.65 V, the battery life with fixture was 598 times, while the battery life without fixture was 119 times, with a difference of 479 times; at 0.50-3.65 V, the capacity retention of the battery without fixture for 360 times was 67.72%, while the battery capacity retention rate with fixture is 90.00%; at 0.80~3.65V, the battery capacity retention rate without fixture is 80.00% for 500 times, while the battery capacity retention rate with fixture is 92.00%;

 

Under 1.50~3.65V, the battery capacity retention rate of 928 times without fixture is 84.55%, while the capacity retention rate of the battery with fixture is 86.73%, which is a capacity retention enhancement of 2.18 percentage points; under 2.50~3.65V, the capacity retention rate of the battery without fixture of 1000 times is 94.07%, while the capacity retention rate of the battery with fixture is 96.03%.

 

As the lower limit of discharge voltage increases, the effect of fixture on the battery cycling performance is gradually weakened, which is manifested by the narrowing of the gap between the capacity retention rate of the battery with fixture and without fixture. This indicates that the enhancement effect of the fixture on the battery cycling performance is weakened in the higher voltage range.

 

The EIS test was performed on the batteries with and without fixtures in the late cycling period of some schemes (4 groups, such as A, B, C, and F), and the results are shown in Fig. 3. The starting point of the semicircle of the impedance spectrum is the interfacial impedance Rb at the interface between the electrode and the electrolyte; the end point of the semicircle is the charge transfer impedance Rct; and the diagonal line is the impedance caused by the diffusion of Li+ inside the active material particles, which is the Warburg impedance (Dw). From Rb and Rct: A-1<A-0, B-1<B-0, C-1<C-0, F-1<F-0, indicating that the interfacial impedance and charge transfer impedance of the cell with fixtures are reduced.

 

This also verifies that external pressure can reduce the electrode interfacial voids and increase the interfacial contact area. Under less pressure, the battery will be compressed and the thickness will be reduced, which can reduce the contact resistance, shorten the Li+ transmission path and reduce the loss of active lithium; in addition, it can also prevent the detachment of positive and negative electrode materials due to gas production during the low-voltage discharge, reduce the irreversible expansion of the battery, maintain the stability of the positive and negative electrodes, and improve the cycling performance of the battery.  The lower the lower limit voltage of discharge, the deeper the degree of lithium release from the negative electrode material, the more serious the battery gas production, the existence of the fixture can well inhibit the loss of Li+ activity due to gas production, and thus improve the cycle performance of the battery. This also explains why the difference in the number of cycles between batteries with and without clamps becomes smaller and smaller when the discharge voltage is gradually increased.

 

In order to further observe the changes in the battery cycling at different voltages, the battery was disassembled to observe the changes in the positive and negative materials, and the SEM plots are shown in Figs. 4 and 5.

 

Morphologically, there is no obvious difference between the positive and negative electrode materials after the over-discharge cycle. The negative electrode shows a lumpy shape and the positive electrode shows a small granular shape, and there is no effect on the morphology of the positive and negative electrode materials with and without the fixture. During the continuous shallow charging and deep discharging process, the cellular parameters of the materials may change, especially for the negative electrode material, and the layer spacing may change.

The negative electrode sheet was analyzed by XRD and the results are shown in Fig. 6. From Fig. 6, it can be seen that although the cell was cycled in different voltage intervals, the negative electrode material after cycling was still graphite and no other new material was produced. In the same voltage interval, the characteristic peaks such as (002) and (100) of the negative pole piece of the fixture-free battery are all shifted to a low angle, indicating that the cell parameters of graphite become larger and the spacing of the crystal surfaces becomes larger. 



This means that the graphite layer spacing of the non-jigged battery is increasing during the non-stop charging and discharging process compared to the battery with jigs. This microscopic change in the material leads to a more severe degradation of the cycling performance, and also explains why the cycling performance of the battery with clamps is significantly better than that of the battery without clamps in the same voltage interval.2.4 Characterization of calendar life change after battery overdischarge In order to further observe the capacity change of the batteries after prolonged over-discharge shelving, the capacity retention and recovery of the batteries after shelving for a certain period of time under discharging to the specified voltages of 0.50V, 0.80V, 1.50V, and 2.50V were investigated respectively, as shown in Table 2.

 

From Table 2, it can be seen that the battery does not show capacity decay even when stored at too low a voltage, and there is not much effect on capacity, and there is no loss of capacity due to overdischarge. From the method, after the battery is over-discharged, shelving it for a period of time to recharge it with a small current can restore and maintain the capacity of the battery.

 

3 Conclusions Based on the characterization of lithium-ion batteries for data centers, the effects of overdischarge with different preload forces were investigated, and the results show that.

By applying 2 kinds of preloads with and without clamps to the batteries, cycling at 6 voltage intervals, including 0~3.65 V, 0.20~3.65 V, 0.50~3.65 V, 0.80~3.65 V and 1.50~3.65 V, as well as the conventional 2.50~3.65 V, regardless of whether or not the clamps are present, the lower the cut-off voltage of the battery's discharge is, and the more rapidly the capacity decays. In practical applications, prolonged cycling at too low a voltage should be avoided as much as possible to prolong the service life of the battery.

 

The fixture can improve the cycling performance of the battery, but the effect is weakened with the increase of the discharge cut-off voltage, and the test results of EIS and XRD show that the fixture can ensure the stability of the positive and negative active substances in the cycling process, reduce the electrode interface gap, increase the contact area, reduce the ohmic resistance, shorten the Li+ transport path, reduce the loss of active lithium, reduce the irreversible expansion of the battery, and improve the cycling performance of the battery. performance. The experimental results are of great significance for the design of battery assembly, and it is suggested that the subsequent researchers need to consider the design of tight assembly in order to improve the battery performance.

 

The capacity of the battery does not decrease with the decrease of the discharge cut-off voltage after long time storage at different over-discharge voltage intervals. The capacity can be maintained and recovered by small current recharge. This provides an idea to solve the problem of battery performance degradation after long time storage.