Start and stop the battery "noisy revolution", how to turn over the lithium titanate battery?

Starting and stopping the battery for the battery, its basic requirement is that the battery has a high magnification, a wide temperature range and a long cycle life. At present, 90% of the start-stop batteries on the market are occupied by lead-acid batteries, but lead-acid batteries still have some problems, such as unfriendly environment, poor low-temperature discharge performance, low energy density, and poor cycle life.

As an alternative to lead-acid batteries, lithium-ion batteries also face challenges such as high rate charge and discharge performance, safety and cost. The negative electrode of lithium ion battery in the market is basically graphite. However, there are several problems with graphite anode. Firstly, the lithium ion diffusion coefficient of graphite is relatively low, so the yield performance of graphite anode lithium ion product is not particularly ideal.

The second is the cycle life. The graphite anode will undergo a certain volume expansion during the process of deintercalating lithium, generally about 10%. At the same time, the SEI film formed on the surface of the graphite anode is a dynamic change of continuous consumption and repair during charge and discharge. Process, so the problem of cycle capacity decay of batteries using graphite anodes has yet to be overcome;

The third is safety considerations. The graphite anode deintercalation potential platform is about 0.1-0.15V, and it is easy to form lithium precipitates at low temperature or large magnification, which leads to safety accidents. Another point is that the lithium-incorporated graphite anode itself is a very active substance. It has a high level of insecurity when the battery is out of control.

As a substitute for graphite anode, lithium titanate anode has the following advantages. First, it has a higher lithium ion diffusion coefficient. At present, according to different synthesis methods of lithium titanate, the lithium ion diffusion coefficient can be 10-7 to 10- 9, thereby ensuring the high rate performance of this material of lithium titanate;

Second, lithium titanate has an extremely long cycle life. Since the size of the crystal lattice changes very little during the process of deintercalating lithium from lithium titanate material, it is known as a zero strain material, so the cycle life is very long;

Thirdly, in terms of safety, lithium titanate has a deintercalation potential of about 1.55V for lithium. It is difficult to form lithium dendrites under charge and discharge and low temperature. lithium titanate itself is a relatively stable substance, and it is unique in terms of safety advantage.

Chaowei Chuangyuan first selects the material system in the design of lithium titanate-based start-stop battery. In the material aspect, it mainly chooses lithium titanate with small particle size, and improves the titanic acid through reasonable coating and doping modification. The lithium ion diffusion coefficient and conductivity of lithium are the main purpose, and it solves the processing problem in the manufacturing process of the battery core.

In terms of electrolyte and porous film, we have also made some choices. We choose a separator with high porosity and high strength. The electrolyte takes into account the conduction rate of lithium ions at normal temperature and low temperature.

In terms of starting and stopping the battery test, Chaowei Chuangyuan's enterprise standard is based on the VDA standard and the national standard 31484-6. In some key indicators, our corporate standards are stricter than the VDA standard, such as low temperature performance and Cycle life safety testing is carried out with reference to the national standard.

At the same time as the development of the battery core, we also carried out a thermal simulation model analysis of the battery core. In cooperation with the Institute of Chinese Academy of Sciences, this is only a part of the cooperation project. From the preliminary modeling analysis, the lithium ion diffusion by the lithium titanate material Coefficient, conductivity, specific heat energy, density, specific surface area, particle size and other factors, the temperature field distribution of the cell at the end of 7C rate discharge is relatively stable, the internal temperature difference is less than 4 ° C, and the 7 C discharge maximum temperature The rise is less than 7 ° C, which is close to our actual test results.

Our battery capacity is 20Ah, 75.6% capacity can be released at minus 20 °C, 70% capacity can be released at minus 30 °C, 11C charging can be charged 97% capacity, and 7C discharge capacity retention rate is 88. %. Stored at room temperature, the charge retention rate after 97 days is 97.7%. The 1C charge and discharge cycle was 8000 times, and the capacity retention rate was 98.5%. 3C was charged and discharged 6,000 times, and the capacity retention rate was 91%.

In terms of battery safety testing, we strictly test according to GB/T31485. In addition, we have added some more rigorous testing items, such as 180° bending, 150°C hot box experiment, and fire burning experiment. For the low temperature start of the battery, the VDA standard country is minus 18 ° C, 11 C discharge is 10 s, and the terminal discharge voltage is required to be greater than 1.2 V, the standard we use is minus 20 ° C, 11 C discharge for 10 s, and the same terminal discharge voltage is required to be greater than 1.2 V.

We intercepted two data, 80% SOC and 100% SOC low temperature start-up test, 80% SOC minus 20 ° C discharge end voltage is 1.38V, 100% SOC end discharge voltage is 1.54V, we conducted a cyclic test according to the VDA standard, tested 50% and 17.5% DOD cycle test, 50% of the DOD cycle test method is 3C charge and discharge, charge and discharge capacity is 11Ah, charge and discharge interval is 50% - 100% SOC, cycle The cut-off condition is that the cycle is terminated when the terminal discharge voltage is lower than 1.5V.

We tested the 1500-week cycle, the terminal voltage of the battery dropped by approximately 19 mV, and the terminal discharge voltage was maintained above 2.3V. For the 17.5% DOD cycle, the 3C charge and discharge is also 3.85Ah, and the cycle is terminated when the terminal voltage reaches 1.5V. So far, 2500 cycles have been performed, and the terminal voltage has dropped by about 13mV. The capacity attenuation is very small.

Here again, our lithium titanate battery has high rate performance. One of the main reasons is that the memory is relatively small, the AC internal resistance is about 0.6mΩ, the 11C charge capacity retention rate is 97%, and the low temperature start-stop test is minus 20 When the °C@11C discharges for 30s, the terminal voltage is greater than 1.5 volts, and the sub-20 °C @1C discharge capacity is maintained at 72%.

In terms of safety testing, in addition to testing according to the national standard GB/T31485 test items, we have also added some more rigorous testing items, such as 150 ° C hot box storage experiment 180 ° bending experiment and fire test.

We also did extreme degradation experiments, fire test, flame temperature between 600-700 ° C, in the early stage of combustion inevitably there is a combustion of the diaphragm and electrolyte, the flame has a growing trend, but at the moment of growth we After the battery is removed, the flame is extinguished immediately, indicating that the battery itself is not flammable except for the electrolyte and the diaphragm. The safety of other main materials of the battery itself is still very good. In the future, we will focus on the improvement of the safety of the battery cells in the electrolyte and diaphragm to achieve the development of a completely non-combustible start-stop battery.

Many of our work have taken into account the development of start-stop batteries and HEV batteries. There have been a lot of other test items for lithium titanate-based batteries, which are not covered here.

Finally, a summary, lithium titanate-based start-stop battery has an extremely long life, more than 10,000 times. Our super-powered start-stop battery passed the 150°C hot box storage experiment and the 180° bending test, which showed that it has very good safety, and there is still some room for improvement afterwards, mainly in terms of electrolyte and diaphragm. Realize the development of start-stop batteries that are completely non-combustible. The battery itself has ultra-high power performance and can charge 97% of the capacity in the case of 11C charging.

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