Is low temperature the biggest weakness of lithium batteries?
In tests conducted by the American Automobile Association (AAA), an electric vehicle had a range of 105 miles (about 169 kilometers) at 75 degrees Fahrenheit (about 24°C), but this dropped to 43 miles (about 69 kilometers) at 20 degrees Fahrenheit (about 7°C)—a decrease of 60%. Batteries are somewhat similar to humans; they become less active in colder weather. Lead-acid batteries, lithium-ion batteries, and fuel cells are all affected by low temperatures, albeit to varying degrees.
Taking lithium iron phosphate (LFP) batteries, the most commonly used batteries in electric buses, as an example, this type of battery boasts high safety and long single-cell lifespan, but its low-temperature performance is slightly inferior to batteries using other technologies. Low temperatures affect the positive and negative electrodes, electrolyte, and binders of LFP batteries. For instance, the positive electrode of LFP itself has relatively poor electronic conductivity, making it prone to polarization at low temperatures, thus reducing battery capacity. Low temperatures also slow down the lithium intercalation rate in graphite, making it easier for metallic lithium to precipitate on the surface of the negative electrode. If the battery is put into use without sufficient resting time after charging, the metallic lithium cannot be completely re-intercalated into the graphite, leaving some metallic lithium on the surface of the negative electrode, potentially forming lithium dendrites and affecting battery safety. At low temperatures, the viscosity of the electrolyte increases, and the lithium-ion migration resistance also increases accordingly. Furthermore, binders are a crucial factor in the production process of LFP batteries, and low temperatures significantly impact their performance.
While both are lithium batteries, lithium titanate batteries exhibit superior low-temperature performance. The spinel-structured lithium titanate anode material has a lithium intercalation potential of approximately 1.5V, preventing the formation of lithium dendrites, and exhibits a volume strain of less than 1% during charge and discharge. Nanoscale lithium titanate batteries can be charged and discharged at high currents, achieving fast charging at low temperatures while ensuring battery durability and safety. For example, Yinlong New Energy, a company specializing in lithium titanate batteries, has products capable of normal charge and discharge at temperatures ranging from -50 to 60°C.
Although lithium-ion batteries with graphite as the negative electrode can discharge at -40°C, it is difficult to achieve conventional current charging at -20°C and lower temperatures, which is an area that the industry is actively exploring.
Industry exploration of low-temperature resistant lithium batteries
Industry companies and research institutions have focused their efforts on improving the low-temperature performance of batteries by modifying the processes of existing positive and negative electrode materials and creating conditions for batteries to operate at low temperatures by increasing the local ambient temperature of the battery.
Current battery materials are moving towards nanoscale manufacturing. The particle size, resistivity, and AB plane axis length of the material all affect the battery's low-temperature characteristics. Wotema has prepared lithium iron phosphate materials using three different processes, employing different techniques for nanoscale formation and coating. Results show that increasing the AB plane axis length enlarges the lithium-ion migration channels, which is beneficial for improving the battery's rate performance. Among the materials produced by the three processes, graphite particles with larger interlayer spacing exhibit lower bulk impedance and ion migration impedance. Regarding the electrolyte, Wotema, based on a fixed solvent system and lithium salt, uses low-temperature additives to increase the discharge capacity from 85% to 90%. It is understood that as early as the end of 2016, Wotema had already achieved a 0.5C charging constant current ratio of 62.9% in environments of -20, -30, and -40℃, and a discharge capacity of 94% at -20℃. Currently, Wotema's low-temperature batteries have been widely deployed in Inner Mongolia, the three northeastern provinces, and other regions.
On August 31st, a research team including Beijing Institute of Technology announced the successful development of an all-weather battery product. Technicians utilized the principle of heat generation through energized metal wires, adding nickel foil to the battery cell. The nickel foil generates heat when energized, raising the internal temperature of the battery. Once a certain temperature is reached, the foil automatically disconnects to ensure battery safety. It is understood that in an experimental environment of -30℃, the battery using this technology can rapidly heat up to above 0℃ in 30 seconds, increasing discharge power by more than 6 times and charging power by more than 10 times. Team members stated that this technology does not alter the original battery structure and has extremely low modification costs, making it suitable for various types of batteries, including lead-acid and lithium batteries. According to Battery China Network, an all-weather electric vehicle using this technology will be released at the end of December 2017, with the development of 11 prototype vehicles across four models expected to be completed by 2020, and demonstration operation commencing.
According to media reports, at the 2017 "Maker China" Xinjiang Innovation and Entrepreneurship Competition, which concluded on September 20, Dr. Wang Lei from the Xinjiang Institute of Physics and Chemistry, Chinese Academy of Sciences, led a team that won the first prize in the maker group for their "all-weather lithium battery." This lithium battery can operate stably in environments ranging from -40℃ to 60℃. Currently, the team has completed product testing under various high and low temperature conditions and is about to enter the commercial production stage.
On September 19, 2017, 70 12-meter gas-electric hybrid buses equipped with Microvast MpCO lithium batteries officially went into service in Baotou, Inner Mongolia. The region experiences temperatures ranging from below -30°C to as high as 39°C. Baotou chose Microvast's fast-charging battery system precisely because of the excellent environmental adaptability of Microvast's fast-charging batteries.
Shandong Weineng is a high-tech enterprise specializing in the research and development and production of special low-temperature lithium iron phosphate batteries. In cooperation with the Institute of Chemistry of the Chinese Academy of Sciences, the lithium iron phosphate batteries developed and produced have achieved a major breakthrough in low-temperature performance, and can release more than 90% of the rated capacity at a low temperature of -40℃.
Furthermore, Penghui Energy's power batteries can be used in environments ranging from -20°C to 60°C without the need for heating or cooling systems. Sunton New Energy's ternary lithium batteries have significantly improved low-temperature performance, with cells capable of normal discharge at -20°C, meeting the needs of many vehicle manufacturers.
Why does charging require more temperature than discharging?
Attentive readers may notice that many companies' battery products can discharge normally at low temperatures, but struggle to charge normally, or even fail to charge at the same temperature. Why is that?
According to industry insiders, when Li+ ions intercalate into graphite materials, they first undergo desolvation, a process that consumes energy and hinders the diffusion of Li+ ions into the graphite interior. Conversely, when Li+ ions exit the graphite material and enter the solution, they undergo a solvation process, which consumes no energy, allowing Li+ ions to quickly exit the graphite. Therefore, the charge acceptance capability of graphite materials is significantly inferior to their discharge acceptance capability.
Charging batteries at low temperatures carries certain risks. As the temperature decreases, the kinetic properties of the graphite anode deteriorate further. During charging, the electrochemical polarization of the anode intensifies significantly, and the deposited lithium metal is prone to forming lithium dendrites, which can penetrate the separator and cause a short circuit between the positive and negative electrodes.
Therefore, industry experts recommend avoiding charging lithium-ion batteries at low temperatures as much as possible. When batteries must be charged at low temperatures, it is necessary to choose a small current (i.e., slow charging) to charge the lithium-ion batteries, and after charging, let the lithium-ion batteries rest sufficiently to ensure that the metallic lithium deposited at the negative electrode can react with the graphite and re-intercalate into the graphite negative electrode.
Of course, lithium titanate batteries have material advantages; they can still achieve fast charging at low temperatures, a capability that other battery materials can hardly replicate.
