What is the impact of the lithium battery aging system on battery performance?

In this paper, the lithium battery is heated under the low temperature condition by itself, and a more accurate temperature rise model is discussed. The discharge current, state of charge and temperature rise rate are integrated into a relational formula. The conclusions of the literature are as follows:

“The results show that the discharge rate and heating time are exponentially decreasing, similar to the discharge rate and power consumption. When the 2 C discharge rate is selected, the battery temperature can rise from -10 ° C to 5 ° C in 280 seconds. In this case, the power consumption of the heating process does not exceed 15% of the rated capacity. As the emission rate gradually decreases, the heating time and power consumption of the heating process increase slowly. When the discharge rate is 1 C, the heating time exceeds 1080 s. The consumption is close to 30% of the rated capacity. The effect of the heat release rate on the heating time and power consumption during the heating process is significantly enhanced at less than 1 C. When the discharge rate is 1 C, the heating time exceeds 1080 s, and the power consumption is close to the rated capacity. 30%. The effect of heat release rate on heating time and power consumption during heating is significantly enhanced at less than 1 C. When the discharge rate is 1 C, the heating time exceeds 1080 s, and the power consumption is close to 30% of the rated capacity. The effect of heat rate on heating time and power consumption during heating is significantly enhanced at less than 1C."

The basis of the discussion is that lithium batteries are charged at a low temperature. There is an exact and unacceptable hazard of lithium deposition, impaired cycle life and increased risk of thermal runaway. In the case of low-temperature discharge, in addition to the temporary decrease in discharge capacity (after the temperature rises, it is considered that this part of the capacity will rise again), there is no other explicit hazard. Is the low temperature 2C discharge really without any permanent hazards? If there is a hazard, it is necessary to consider the effects of accelerated aging caused by self-heating of the discharge on the entire function and cost system. Today's paper focuses on the effects of temperature and discharge rates on cell aging rates.

First of all, the conclusion of this article: Lithium-ion battery discharge has an optimal operating temperature, which is the temperature with the lowest decay rate. Above or below this temperature will have an impact on battery life. Here we need to pay attention to a premise, different types of batteries are most suitable for different temperatures, so high temperature and low temperature are relative values. This paper is a series of tests and discussions on "LiCoO2/LiNi0.8Co0.15Al0.05O2 soft-packed lithium-ion battery". The details are as follows, this time it is mainly split into two parts.

1 Introduction

In order to improve the reliability of lithium-ion battery applications in the automotive field, it is important to understand its aging behavior. In the past few decades, efforts have been made to explain the aging behavior of lithium ion batteries. Wang based on graphite LiFePO4 to establish a cycle life model including temperature, depth of discharge (DOD), and discharge rate. [1] Ecker et al. developed a semi-empirical calendar aging model based on the temperature and state of charge (SoC) of a graphite-LiNi 1/3 Mn 1/3 Co 1/3 O 2 battery. [3] Nevertheless, due to the existence of a variety of electrode materials, battery structure and electrolyte composition, people still have a smattering of the aging behavior of lithium-ion batteries.

The aging of lithium-ion batteries depends not only on the time or number of cycles, but also on the operating conditions, ie the stress factors. An in-depth analysis of the effects of decisive stress factors including temperature, charge and discharge rate, DOD and average SOC is a prerequisite for extending the life of lithium-ion batteries and ensuring their performance reliability.

Temperature has a large effect on the cycle aging rate of lithium ion batteries. The lower temperature lowers the cycle life due to the enhanced lithium elemental plating; the excessive temperature lowers the battery life due to the Arrhenius-driven aging reaction; therefore, the lithium battery can only achieve the optimum cycle life at the proper temperature. [6] W aldmann7 conducted a comprehensive experiment covering a temperature range from - 20 ° C to 70 ° C, and found that the 25 650 is a LixNi1/3Mn1/3Co1/3O2 / LiyMn2O4 mixed cathode and graphite / carbon anode 18650 type to obtain the longest battery cycle life The best working temperature. As shown by other research work, the optimum cycle temperature may not be 25 °C. There are many different types of batteries, and the optimal cycle temperature is not the same. The optimum temperature obtained by Schuster et al. [5] was found to be 35 ° C, while Bauer et al [8] detected an optimum temperature of about 17 ° C. Temperatures above the optimum cycle temperature accelerate the formation of the solid electrolyte interface (SEI), resulting in rapid capacity decay and impedance rise. The lower temperature at the end of the charging process facilitates the formation of lithium plating on the surface of the negative electrode. Many researchers [9–13] have confirmed the presence of lithium plating on the negative electrode of lithium batteries using in-situ or ex-situ methods, but no one has yet to clearly report on the problem of cathodic degradation.

The discharge rate is reported to have an exponential effect on the aging rate of lithium ion batteries. Cui et al. determined the relationship between the discharge rate and the capacity loss of the equation 1.15 Ah LiCoO 2 / MCMB (mesophase carbon microbead) lithium ion battery.

Here, Qloss is the capacity loss, T is the absolute temperature in Kelvin, C is the discharge rate, n is the number of cycles, A(C) is the pre-exponential factor, and Ea(C) is the activation energy.

Omar et al. [16] also reported the exponential effect of discharge rate on the cycle life of a cylindrical 2.3 Ah LiFePO 4 /graphite lithium ion battery. Wang et al. [1] extracted the battery life model similar to that of Cui et al. above, and the relationship between discharge rate and capacity loss, as shown in the following equation [4]. The results are based on a large number of 26650 cylindrical LiFePO 4 / graphite lithium ion battery cycle test data.

What is the impact of the lithium battery aging system on battery performance?

Where Qloss is the capacity loss, B is the pre-exponential factor, C Rate is the discharge rate, R is the gas constant, T is the absolute temperature in Kelvin, and Ah is the amount measured in Ah. Equations [1] and [4] are empirical models, so the units on either side of the equal sign are not exactly the same.

Many researchers believe that high current discharges can cause cracks in the SEI layer, followed by SEI repair. [1, 14, 16, 18, 19] Therefore, the side reaction on the surface of the anode was accelerated, and the thickness of the SEI film was further increased. All of these processes increase the consumption of recyclable lithium and the impedance of the battery. In fact, higher battery temperatures are always accompanied by higher discharge rates, which obscures the real cause of accelerated battery aging at high discharge currents. In this paper, the effects of stress factor temperature and discharge rate on mixed cathode lithium ion soft pack batteries were investigated.

The hybrid cathode was developed to combine the advantages of different cathode materials. Some research groups have tried to explain the aging mechanism of the mixed cathode LiMn2O4/LiNi1/3Mn1/3Co1/3O2. [2,20,21] They have found that the aging mechanism in such cells is mainly the loss of recyclable lithium and partial loss of cathode material. However, little information is available about the aging behavior of the LiCoO2/LiNi0.8Co0.15Al0.05O2 (LCO/NCA) hybrid cathode.

2 test

In order to study the effects of temperature and discharge rate stress factors on lithium-ion batteries, a SLPB50106100 lithium-ion soft pack battery with a nominal capacity of 5 Ah from the manufacturer Kokam was tested. According to the results of energy dispersive X-ray spectroscopy, the active material of the battery is composed of graphite at the anode and LCO / NCA at the cathode. The data sheet gives the range of parameters from 2.7 V to 4.2 V and the maximum current rates of charging and discharging 2C and 5C.

In our aging experiments, performance tests are performed periodically to check the health of the cells (SoH). Performance testing is divided into basic performance testing and extended performance testing. The basic performance test, the capacity test, is performed every two weeks. Extended performance testing is performed every four weeks, including capacity testing, open circuit voltage (OCV) testing, and electrochemical impedance spectroscopy (EIS) testing. Cyclic testing, capacity testing and OCV testing are managed by BaSyTec's Battery Test System (CTS). The EIS test was performed on a potentiostat VMP3 from Biologic Science Instruments. All of these tests were performed in a climate chamber at 25 ° C. The cell test was conducted under normal atmospheric pressure without applying additional external pressure.

In the capacity test, the remaining capacity is measured as follows. A constant current (CC) of 1 C (corresponding to 5 A) was charged to 4.2 V, and then switched to a constant voltage (CV) at 4.2 V. In the CV phase, when the current drops below 0.05C, the cell is considered to be 100% full. After a 10 minute pause, a 1 C CC was applied to discharge the battery to 2.7 V, followed by the CV phase, and further discharged until the current dropped below 0.05 C. The purpose of this CV phase is to minimize the effect of impedance rise in the cell on the measured capacity. The OCV test is always started 10 hours after the above capacity test to rule out the effects of the OCV curve rebound. A 0.1 C CC was implemented to charge the battery to 4.2 V, and then the same CV charging phase as described above was used. After 1 hour of pause, the battery was discharged to 2.7 V at 0.1 C CC and discharged in the same CV discharge phase as described above. After 6 hours of OCV testing in constant current mode, the impedance spectrum of the cell at 50% SoC was measured with an AC amplitude of 200 kHz to 10 mHz AC excitation. The real part of the impedance at the zero crossing in the Nyquist diagram is taken as the ohmic resistance of the battery.

Table I provides an overview of the aging test matrix. Temperature test series, select 10 °C, 25 °C and 40 °C. On the cycle curve, at each temperature, the cells were charged with the 1C CC-CV program and discharged using the 1C CC program. The charging process switches from CC to CV at 4.2V, and when the current drops below 0.05C, the CV process stops. The discharge process stops at 2.7V. For the discharge rate test series, all cells were tested at 25 °C. The discharge process was changed to 3C and 5C CC discharges, while the charging process remained unchanged. Test at least two cells under each aging condition and give their average performance and range of maximum and minimum values in the following sections.

Table I. Aging test matrix for investigating stress factor temperature and discharge rate

What is the impact of the lithium battery aging system on battery performance?

What is the impact of the lithium battery aging system on battery performance?

Figure 1. Performance test results at 25 °C: (a) normalized capacity is plotted as a function of the equivalent complete period, and (b) normalized ohmic resistance is plotted as a function of the equivalent complete period. The cell data is displayed at 1 C at 10 °C, 25 °C and 40 °C cycles.

3 Results and discussion

In order to study the aging behavior of LCO / NCA-graphite based batteries, the evolution of battery performance parameters, namely discharge capacity and ohmic resistance, was performed under all operating conditions to extract and compare discharge capacity and ohmic internal resistance. In addition, EIS, differential voltage analysis (DVA) and incremental capacity analysis (ICA) are used as aging detection methods to reveal relevant aging mechanisms.

Temperature effect

This section describes the aging behavior of test cells at different temperatures. Figure 1a shows the normalized discharge capacity and equivalent full cycle (EFC) at three ambient temperatures. Table 2 lists the minimum, maximum and average values for the surface temperatures of different circulating batteries. The three ambient temperatures, 10 °C, 25 °C and 40 °C, correspond to the measured temperatures of the three cell surfaces at 10.1 °C, 27.5 °C and 41.5 °C, respectively. These temperature data are taken from the last cycle before the battery capacity loss reaches 20%. These values can be considered as worst case values because the internal resistance of the battery is lower in earlier cycle tests. Initially, the average discharge capacity (CC + CV) of all test cells was 5.709 Ah with a deviation of ± 0.26%. In the first 300 cycles, all cells showed a basic capacity decline. After that, different cell capacities decrease linearly at different rates. The cycle was carried out under mild conditions at 25 °C to obtain the lowest rate of capacity decay. Both 10 ° C and 40 ° C cycles accelerate the decay of capacity. Testing the capacity deviation of different batteries is usually negligible under the same load conditions, demonstrating excellent battery quality. These batteries are an exception when the cycle capacity loss at 40 °C exceeds 15%. Here, two different aging curves were observed. One unit continues to lose capacity linearly on the EFC trend. The other unit shows a doubling effect of battery capacity decay.

Table 2 Surface temperature of the battery in the last cycle test before the battery capacity loss of 20%

What is the impact of the lithium battery aging system on battery performance?

Figure 1b shows the relationship between normalized ohmic resistance and EFC. The data comes from EIS measurements. At the beginning, the average ohmic resistance was 2.7 mΩ with a deviation of ± 5%. In all cases, the ohmic resistance increased linearly from the beginning. Contrary to the capacity decay in Figure 1a, the increase in ohmic resistance is exacerbated with increasing temperature, indicating that different aging mechanisms result in increased capacitance attenuation and ohmic resistance. The resistance deviation on the EFC is greater than the capacity deviation and gradually increases. In the case of cells that cycle at 40 ° C, the evolution of resistance is also divided into two modes, one with a continuous linear increase and the other with an increase. This is consistent with their trend in capacity changes.

The linear dependence of battery capacity on EFC has been reported in many studies. [4, 20, 22] The aging mechanism in this linear region can be classified into cycle-induced capacity loss and capacity loss based on calendar aging. The capacity loss caused by the cycle refers to the lithium consumption caused by the cyclic trigger crack on the anode particles and the additional SEI formation. Capacity loss based on calendar aging is associated with chemical parasitic acceleration of temperature. Lithium-consuming reactions, such as the formation and reconstruction of SEI. The capacity decay rate at 10 °C is higher than that at 25 °C. It is likely to be the result of lithium plating [7, 8]. Due to the lower temperature, the internal electrode resistance increases and the anode potential eventually drops to a negative potential. The Li / Li + negative reversible potential is reached. [twenty three]

Ohmic resistance originates from the volumetric chemistry of the battery, including the electrical resistance of the electrolyte, active material, and current collector. [5,24 – 27] The increase in ohmic resistance is mainly due to the decomposition of the electric salt and solvent in the electrolyte, which in turn changes the conductivity of the electrolyte. [21,28 - 30 ]

At the same time, a cell that circulates at a higher aging rate at 40 ° C and a new cell are disassembled. It was found that the inside of the aged cell was dry because no trace of the electrolyte wetted electrode and the separator was observed. There is no visible liquid electrolyte, and it can be assumed that electrolyte decomposition is the cause of significant capacity decay and resistance increase. In addition, electroplated lithium was also observed on the aged anode layer, at a temperature of 40 ° C, which is usually not thought of.