Editor's note
The effect of low temperature on lithium-ion power batteries and PEMFC stacks is very different. Relatively speaking, low temperature has a greater negative impact on the performance of lithium-ion batteries.
2.4 Comparison of low temperature performance of lithium battery and fuel cell
Due to the extensive use of the car, low temperature performance is an indispensable technical indicator for power batteries. The low temperature performance of lithium battery depends mainly on the influence of temperature on the conductance, ion diffusion coefficient and electrolyte conductivity of the electrode material. The viscosity of the electrolyte at low temperatures increases and the conductivity decreases, resulting in a sharp increase in cell polarization. In general, the performance of a lithium-ion battery is drastically reduced near zero, and -20 oC is almost impossible to work properly. Frequent charging and discharging at low temperatures can seriously deteriorate the life of the power battery, and it is easy to cause lithium leakage from the negative electrode, which poses a safety hazard. In general, improvements to the low-temperature performance of lithium-ion power batteries can have a large negative impact on other technical indicators such as cycle and energy density, and lead to a rise in cell cost.
The low temperature problem of PEMFC is called cold start. The cold start requires the FC-EV to be restarted within a certain period of time after the shutdown. Cold start is one of the technical bottlenecks for the commercialization of FC-EV due to the formation of ice in the PEMFC stack that prevents electrochemical reactions in low temperature environments. However, once the PEMFC is started, due to its own heat release, even at very low ambient temperatures, the temperature of the stack will quickly stabilize at the normal operating temperature range of 80-90oC, which is a big part of the PEMFC and lithium battery operation. the difference.
There are many theoretical and experimental data on the cold start problem below the PEMFC freezing point. At present, Daimler-Benz has already started at -25 oC, Toyota, Nissan and Honda have already started at -30 oC, and the target for cold start of ordinary cars is -40 oC, so FC-EV still needs to further improve cold start capability. .
From the above analysis, we can clearly see that the effect of low temperature on lithium-ion power batteries and PEMFC stacks is very different. Relatively speaking, low temperature has a greater negative impact on the performance of lithium-ion batteries.
2.5. Reliability comparison between lithium ion battery and fuel cell
The reliability of a battery refers to the probability that a battery will cause it to lose its ability to store electrical energy. The reliability of lithium battery has a lot to do with its safety, but it is not the same concept. A safety accident in lithium battery will inevitably lead to its loss of electrical energy storage capacity. However, the loss of electrical energy storage capacity of lithium batteries is not always due to safety accidents, such as battery failure due to capacity "diving".
The power battery system assembles hundreds of individual cells through series and parallel, and the reliability of the entire battery system will be greatly amplified. Judging from the very limited usage data accumulated by domestic pure electric vehicles, the reliability of large-scale power battery systems is relatively unsatisfactory. The root cause of the reliability problem of lithium-ion batteries lies in the safety influencing factors discussed above, and these factors are determined by the essential characteristics of the embedded response.
Before discussing the PEMFC reliability issue, let's look at the actual application of the fuel cell. PEMFC is actually developed on the basis of AFC (Alkaline Fuel Cell) technology. AFC is a power source tailored for aerospace flight. In the 1970s, United Technologies Corporation (UTC) first succeeded in applying AFC stacks to the US space shuttle, followed by the International Fuel Cell Company (IFC). The third-generation AFC (nominal/extreme power 7.0/12.0 KW) produced later became the standard power source for the US space shuttle, showing that the fuel cell technology itself has extremely high reliability.
Another military field that uses batteries on a large scale is conventional submarines. The use of high-energy battery systems in conventional submarines can greatly increase their underwater activity time, which is undoubtedly of great tactical value. At present, most of the conventional submarines in the world are powered by lead-acid batteries. There is no international lithium-ion battery in the case of conventional submarines (main power batteries) or nuclear-powered submarines (auxiliary batteries). The source of rare earth lithium-ion battery is not in the discussion of the author.
The root cause of the Western military powers not including the lithium-ion battery system on their new generation of conventional submarines is obviously not due to price considerations, but mainly because large lithium-ion battery has a large reliability, especially safety. Hidden dangers do not meet the harsh environment of the submarine. As for the latest Japanese "Canglong"-class conventional submarine that was previously reported by the media, lithium-ion batteries will be used for the first time, and later clarified due to technical and budgetary problems, and Japan's next-generation conventional submarines have determined to use PEMFC as AIP (not dependent) Air propulsion device) power. 
Most of the AIP conventional submarines currently in use or about to be in service worldwide use PEMFC as the main power battery system. In 1987, the German Navy successfully used a Siemens reactor (AFC) to successfully rebuild a 206-class test boat, making it the world's first practical fuel cell AIP submarine. Since then, Germany has continuously developed the 212 and 214 two-stage AIP submarines (the PEMFC stack was jointly developed by Siemens and HDW, 2×120 KW) and has now developed to the latest 216.
New conventional submarines in Russia, South Korea, Australia, Israel and Italy all use PEMFC fuel cell AIP technology. We can imagine that, according to the extremely high requirements of the military for reliability and safety, the large PEMFC stack has developed to a high degree of perfection and reliability. In fact, from the development history of AFC and PEMFC, the fuel cell has been developed from the beginning to be positioned in a large "power battery", which has evolved from a small mobile phone battery to a large power battery. The road is totally different.
Above we have compared the lithium-ion battery and fuel cell from several major technical layers. As the author pointed out at the beginning, the secondary battery is an electrical energy storage device, and the fuel cell is a power production device. The most essential difference determines the different positioning of the two in the application field. There is a big difference between the secondary battery and the fuel cell. The energy density of the single cell of the large secondary battery including the lithium ion battery is much lower than that of the small battery using the same electrochemical system. The reason has been discussed before.
The fuel cell is just the opposite. Because of the effective use of auxiliary systems to improve mass transfer, humidification, drainage and temperature, the power density of large fuel cell systems is much better than that of micro fuel cells, which is why space shuttle and AIP. Submarines prefer large fuel cells as the main source of power for their power supply systems. Many different characteristics of fuel cells and secondary batteries have determined that secondary batteries are suitable for medium and small power storage applications, while fuel cells are more suitable for higher power applications.
Therefore, the author personally believes that the positioning of lithium-ion batteries on passenger cars is an auxiliary power device, HEV and PHEV and small pure electric vehicles are its main application areas. The PEMFC fuel cell was developed as a large power source from the beginning, and is a veritable "power battery."
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