In the face of high oil prices and the high-performance driving experience brought by new energy vehicles, people are gradually accepting new energy vehicles. At the same time, the rapid popularization of new energy vehicles has also benefited from the rapid development of lithium battery technology in recent years.
LiMO2 (where M is usually a mixture of Ni, Mn and Co) and LiMPO4 (where M is usually Fe), the two most commonly used types of cathode materials in lithium-ion batteries, constitute the ternary lithium battery respectively And lithium iron phosphate batteries, generally referred to as NMC batteries and LFP batteries.
When these two kinds of batteries are used, they are often diametrically opposite in affecting the characteristics and performance of the battery cells. Therefore, automakers using different battery technologies highlight the advantages of the batteries they choose. So, what exactly is the difference between these two batteries?
For NMC batteries, the positive electrode usually has high energy and high specific capacity, and operating at higher voltages will lead to more oxidative conditions, affecting its working life. Also, high voltage can lead to stability and safety issues in the charged state.
Because under high voltage conditions, some of the possible failure modes of the positive electrode of NMC batteries, including irreversible phase transition, electrolyte oxidation and particle cracking, may occur. Conversely, LFP batteries are more stable and safer than NMC batteries because of their smaller specific capacity, volumetric capacity, and lower operating voltage.
In order to keep the working voltage of NMC battery above 4.2 volts in traditional NMC batteries, the negative electrode will retain enough graphite to maintain this voltage, which makes the positive electrode have unused lithium ions and reduce the battery life. If the charging voltage of the battery is lowered in this configuration, it will result in graphite remaining unused. Studies have shown that batteries operating between 2.5 volts and 3.78 volts have proven to be more advantageous in terms of lifespan than those operating between 2.5 volts and 4.2 volts.
While most battery manufacturers are focused on building using current proven battery technology, Jeff Dahn, a renowned scientist and researcher from Dalhousie University in Canada, is more focused on the batteries the world will need in the next 5, 10 or 15 years.
In 2019, scholars such as Professor Dahn proposed for the first time that EV batteries could last for 1.6 million kilometers before they need to be replaced. Professor Dahn also maintains close cooperation with Tesla on battery research and development. In 2015, Tesla signed a 5-year research partner contract with Professor Dahn’s team, and with the help of its research team, Tesla has achieved many breakthroughs in battery technology.
In 2021, the two parties will renew the contract for five years and continue to maintain a partnership in the research and development of lithium battery technology. Over the years, the group has maintained a partnership with Tesla on battery research to bring longer-lasting, faster-charging, and cheaper batteries to electric vehicles and energy storage.
Recently, Professor Dahn’s research group cooperated with Tesla to conduct research on new ternary lithium batteries. The related paper was published under the title “Li[Ni0.5Mn0.3Co0.2]O2 achieves long-life low-voltage lithium-ion battery as a superior alternative to LiFePO4”, which proves that long-term use of batteries up to 100 years is possible.
This is a landmark research achievement that will lead more researchers to carry out related research on long-life batteries. This work achieves similar positive voltage and full negative utilization in both cell types by detailing how to balance the performance of NMC cells to lower-than-conventional voltage operation and comparing with LFP cells. And it is proved that under lower voltage operating conditions, NMC batteries are considered to be a better alternative than LFP batteries, and can be applied in some scenarios that are usually limited to LFP-containing batteries.
By optimizing the structure of traditional NMC batteries, the researchers prepared a battery of single-crystal Li[Ni0.5Mn0.3Co0.2]O2 cathode material (referred to as NMC532 battery), so that it can operate at a lower voltage of 3.8 volts. . At the same time, the battery performance parameters such as Coulomb efficiency, capacity fading and energy density are superior to conventional batteries under low pressure conditions.
Comparison of discharge capacity and average charge-discharge voltage under different battery combinations
After calculations, the new battery can have a service life of up to 100 years at 25 degrees Celsius. Experiments have shown that the energy density of NMC532 exceeds that of LFP batteries, and the cycle life at temperatures of 40 degrees Celsius, 55 degrees Celsius and 70 degrees Celsius greatly exceeds that of LFP batteries.
Moreover, the electrolyte used in the design of the NMC battery is lithium bisfluorosulfonimide, and the pictures (above) demonstrate that the electrolyte has a high service life at high temperatures, far exceeding the lithium hexafluorophosphate used in conventional batteries.
Schematic diagram of approximate stack energy density for LFP cells and NMC532 cells
The picture shows a schematic diagram of the stack energy density of LFP and NMC532 cells in this study. It can be seen that the NMC532 battery produces a stack energy density of 495Wh/L despite the charging voltage of only 3.8V. If the stack energy density of LFP cells is increased by about 3%, it can approach 440Wh/L, but this value is still smaller than that of NMC532 cells.
In addition to a longer lifespan than LFP batteries, the low-voltage operating NMC batteries used in this technology offer many opportunities to improve future lithium-ion development and related devices using them. For example, the charging speed is accelerated because low-viscosity solvents such as methyl acetate and ethyl acetate can enable fast-charging liquid electrolytes with better electrochemical compatibility, but cannot operate at high voltages because of their low oxidative stability.
The operation of NMC532 cells at low voltages avoids considerable oxidative stress. At the same time, the hybrid LFP+NMC positive electrode is made possible because the NMC532 cell can operate at low voltage. The negative electrode materials contained in the NMC532 and LFP batteries are basically the same, and the synergistic work of the two positive electrode materials at the same low voltage provides the possibility for new batteries combining the advantages of the two materials in the future.
So, when will Tesla put this technology into actual production cars in the future? The researchers say it may be a long time away. Because currently, their new batteries cost more than LFP batteries and may not have the power characteristics required for electric vehicles.
But the technology could be well-suited for long-term energy storage, as the higher cost of the new battery technology would then be offset by the extended service life. However, with the publication of the paper, follow-up research based on this technology will open up more possibilities. After all, new energy vehicles are the best choice in terms of economy, comfort and environmental protection. The huge market will stimulate the rapid research and development of technology.
Overall, the NMC532 battery operating at low voltage has a better lifespan and energy density than the LFP battery. The advantages of this technology should be taken into account when considering factors such as insufficient energy density and low service life of LFP batteries. Of course, this does not mean that LFP batteries should be eliminated, because the factors of cost control and high safety cannot be ignored.
