The influence of low temperature cycle on the performance of 18650 battery

The lithium battery in use process will encounter a different environment. For example, temperature is often below 0 ℃ to 10 ℃ in winter in northern China. When the battery charge and discharge temperature drops below 0 ℃, the lithium battery charge and discharge capacity and voltage will be reduced sharply. This is because the migration rate of the lithium ions decreased in the electrolyte, SEI, graphite particles at low temperature. Such harsh cold environment brings precipitation of high specific surface area of the lithium metal.

High specific surface area of the lithium precipitation is lithium battery failure mechanism in the most critical reasons, but also lead to the safety of battery is an important problem. This is because it has large surface area in particular, lithium metal was particularly active inflammable, high surface area of the dendritic lithium is a little wet air can burn.

With the improvement of the electric car battery capacity in, range of ascension as well as the electric vehicle market share continuously improve, more and more demanding for the safety of the electric car. In low temperature environment, Power battery What are performance will change? What are the safety aspects worthy of note?

1.18650 low temperature cycle experiment and the analysis of the battery apart

will18650 battery(2.2 A, NCM523 / graphite) under A certain charge and discharge mechanism under 0 ℃ low temperature simulation cycle. Charging and discharging mechanism is: CC - CV charging, the charging ratio is 1 C, charge cut-off voltage of 4.2 V, charging cut-off current of 0.05 C, CC after discharge to 2.75 V. Because the batteries are SOH is 70% - 80% is defined as the termination of a battery status (EOL).This experiment choose SOH is 70% when the battery end use. Under the condition of the above battery cycle curve as shown in figure 1 (a), based on the cycle after the batteries and battery pole piece and not circulation of diaphragm LiMASNMR analysis, chemical shift results as shown in figure 1 (b).

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Figure 1. The cell cycle curve and LiMASNMR analysis

Cycle in front of a few cycle capacity at low temperature rise, then a steady decline, a less than 50 cycles SOH has fallen to less than 70%.Apart after test battery found that negative pole piece surface with a layer of silver material, guess for lithium metal in circulation after deposition on the surface of the anode materials. Through two experimental contrast analyses LiMASNMR battery, the result is shown in figure b also further confirmed.

There is a broad peak in 0 PPM, illustrate the lithium exists in SEI, circulation after the battery appeared the second peak in 255 PPM, is likely to be lithium metal precipitation in the formation of the surface of the anode materials. In order to further confirm that whether the lithium dendrite, on the SEM observation, the result is shown in figure 2.

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Figure 2. The SEM analysis results

By comparing the and b in figure, can see diagram b have a thick layer of material, but this matter has not been completely cover graphite particles. SEM further amplification ratio, observe D figure to see the thick thin sheet material with needle between material, this material may be high specific surface area of the lithium (also is often said that the dendritic lithium).In addition, the lithium metal deposit is to grow in the direction of the diaphragm, its thickness can be observed through the comparison with the thickness of graphite layer.

What form of sedimentary lithium depends on many factors. Such as the surface of the disorder, charging status, temperature, current density, electrolyte additives, electrolyte composition, applied voltage and so on. Among them, under the condition of low temperature cycle and high current density is the most easy to form a dense high specific surface area of the lithium metal.

2. The battery pole piece thermal stability analysis

Researchers using TGA in battery pole piece and not circulation after the battery pole piece, as shown in figure 3.

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Figure 3. The negative and the positive pole piece TGA analysis (a. b. negative positive)

By above can see, unused electrodes appear respectively in three important peak T material 260 ℃, 450 ℃ and 725 ℃, in this a few location appeared severe decomposition reaction, evaporation or sublimate. And after using electrodes, at 33 ℃ and 200 ℃ the obvious quality loss. The decomposition reaction under low temperature is caused by decomposition of SEI film, also related to factors such as electrolyte composition, of course. High specific surface area of the lithium metal precipitation lead to a lot of the SEI film formed in lithium metal surface is battery under low temperature cycle is one reason why a large number of quality loss.

SEM and circulation after the experiment cannot see any change in the anode materials on the morphology, TGA analysis that in higher quality loss at above 400 ℃.This mass loss may be caused by reduction of the anode material of lithium. As shown in figure 3 (b), as the battery aging, sliding is gradually reduce the content of Li in the anode.SOH100 loss of 4.2%, the positive pole piece of the % % SOH70 the positive pole piece of the mass loss of 5.9%.All in all, after low temperature cycle regardless of the positive pole piece or negative pole piece, its quality loss rate are increased.

3. The electrolyte electrochemical aging analysis

The influence of low temperature environment on the battery electrolyte is by GC/MS analysis. Were never aging cells and retrieve electrolyte samples after aging, GC/MS analysis results as shown in figure 4.

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Figure 4. GC/MS and FID - MS test results

Not low temperature cycle battery electrolyte containing DMC, EC and PC, and FEC, PS, SN as additive to improve battery performance. In not circulation and circulation after the battery, the amount of DMC, EC, PC is constant, line cycle after the electrolyte additives in SN (to restrain high voltage anode electrolytic liquid oxygen decomposition) decreases, so the positive local charge under the low temperature cycle is the cause.BS and FEC is SEI film forming additive, and promote the formation of a stable SEI, FEC also can improve the battery cycle stability and coulomb efficiency.PS anode SEI can enhance thermal stability. Can be seen in the figure, the amount of PS does not decrease with cell aging. FEC has sharply reduced, SOH was 70% do not even see the FEC.SEI FEC disappeared because of continuously rebuilt, and repeatedly rebuilding SEI is Li continuous precipitation caused in the anode graphite surface.

Cell cycle after DMDOHC is the main product of the electrolyte, the synthesis is consistent with the formation of the SEI.In figure 4. A lot of DMDOHC means the large area of the formation of SEI.

4. Without low temperature cycle thermal stability analysis of the battery

Under the adiabatic condition and HWS o model, not low temperature cycle on the battery of battery and low temperature circulation ARC (accelerating calorimeter) test, from the ARC - HWS, exothermic reaction is caused by the internal battery, and has nothing to do with the external environment temperature, the reaction inside the battery can be divided into three phases, as shown in table 1.

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In the process of diaphragm and battery explosion thermalization process, there will be a part of the absorption of heat, but the diaphragm thermalization endothermic is very low, for the whole SHR can be ignored. Initial exothermic reaction from SEI decomposition, and then thermal induced by lithium ion embedded, electron can reach the surface of the graphite, then electronic reduce SEI re-established. Thermal stability of test result is shown in figure 5.

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Figure 5. The ARC - HWS SOC result (a) 0%;(b) 50% SOC;(c) 100% SOC; Dotted line for the exothermic reaction initial temperature, thermal runaway starting temperature and temperature of the thermal runaway

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Figure 6. The ARC - HWS results explain a. temperature of thermal runaway, the biggest ID starts, c. d. thermal runaway initial temperature exothermic reaction initial temperature

Not low temperature cycle battery initial exothermic reaction (OER) began in 90 ℃, near to line up to 125 ℃, and with the decrease of SOC indicated OER extremely dependent on the state of lithium ion in anode. For the battery discharge process, the decomposition reaction among the highest in SHR (since the heating rate) is about 160 ℃, the SHR decreases at high temperatures, thus determines the intercalating the consumption of lithium ion in the cathode.

As long as there is enough lithium ion anode can guarantee the destruction of the SEI can be rebuilt. The anode material thermal decomposition can release oxygen, oxidizing reaction with electrolyte, eventually lead to the behavior of the thermal runaway. Under the high SOC, the anode material in high lithium state, at this time of the anode material structure is also the most unstable. The thermal stability of the consequent is battery declines, oxygen release a quantity to increase, between the anode and electrolyte in the heat of reaction is dominant.

5. In the process of gas energy release

Through the analysis of circulation after the battery, you can see in the vicinity of 32 ℃ SHR started to grow in a straight line. With the release of energy, in the process of producing gas is mainly caused by the decomposition reaction, at first the general task is thermal decomposition of the electrolyte.

High specific surface area of lithium metal anode materials in the surface precipitation, can through the following equation is formulated.

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Cp in public for specific heat, delta T on behalf of the decomposition reaction in the ARC tests battery since the heating temperature rise caused by combined.

In the experiment of the ARC, not circulation between 30-120 ℃ battery heat capacity are tested. Exothermic reaction occurred in 125 ℃, and the battery in the discharge state, there is no other exothermic reaction to the disturbance. In this experiment, the CP has a linear relation with temperature, the following equation.

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The amount of energy release in the reaction can be through to the specific heat of integral, each battery under low temperature heat aging can release 3.3 Kj. The energy released in the thermal runaway process can't work out.

6. Acupuncture experiment

In order to confirm the battery aging battery short circuit experiment, the influence of acupuncture experiment was carried out. The experimental results as shown in the figure below:

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Acupuncture as a result, a is in the process of acupuncture battery surface temperature, b is able to achieve the temperature of the largest

The figure shows the aging of the after completion of discharge of battery and new batteries (SOC0 %) after acupuncture experiments, the tiny difference of only 10 ℃ ~ 20 ℃.For after aging cells, under the condition of adiabatic, 35 ℃ to its absolute temperature T material, it has to do with SHR material 0.04 K/min is consistent.

Not the battery SOC of aging was 50% after 30 s reached the maximum temperature of 120 ℃, the release of the temperature of the joule heat is not enough to achieve this, more than the number of thermal diffusion SHR. Aging in SOC was 50%, after the battery has a certain delay for thermal runaway, sharply when the needle into the battery temperature to 135 ℃.At above 135 ℃, SHR increase caused the thermal runaway of the battery, the battery surface temperature to 400 ℃.

New battery charge for acupuncture experiment is a different phenomenon, some cells directly heat out of control, some cell surface temperature lower than 125 ℃, no thermal runaway. One piece of direct thermal runaway battery after the needle into the battery, the surface temperature reached 700 ℃, melting lead to aluminum foil, after a few seconds a column melted away from the battery and then lit out of gas, resulting in the red. Can assume that two sets of different phenomenon appears at 135 ℃ for the diaphragm to melt, when the temperature higher than 135 ℃, the diaphragm is melting and internal short circuit, produce more heat eventually lead to thermal runaway. In order to verify the matter, will not thermal runaway battery to disassemble, and evaluate the AFM in the diaphragm. Result shows that occurred on the diaphragm on both sides of the diaphragm to melt the initial state, yet still appeared in the cathode side of the porous structure, and have not found in the anode side.

7. To summarize

18650 battery under the low temperature circulation, prone to high specific surface area of lithium metal precipitation, lithium metal shape is related to such factors as the current density and voltage. To not cycle battery and low temperature cycle at the same time after the battery for stability, such as acupuncture experimental research, may draw the electrolyte plays an important role in it. Low temperature cycle inside the battery electrolyte will produce other decomposition reaction, so after low temperature cycle batteries more unstable, and mechanical abuse will improve its security risks.

In order to strengthen the safety of the battery, the need to reduce the electrolyte and electrode, the reaction between the electrode and the electrolyte contact interface is an important research content.

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