(Full size image of MWNO crystal structure. Red, green, gray (in light green octahedron) and purple spheres correspond to O, Nb, W and Mo atoms in the cell, respectively. The structure is composed of 4 ×? 4 ReO3 block composition.)
Researchers from the Oak Ridge National Laboratory (ORNL) of the US Department of Energy and the University of Tennessee, Knoxville have found a key material for fast charging lithium-ion batteries. This method with commercial value opens up a potential way to improve the charging speed of electric vehicles.
Lithium ion batteries, or LIBs, play an important role in the country's clean energy technology portfolio. Most hybrid electric vehicles and all electric vehicles use LIBs. These rechargeable batteries have advantages in reliability and efficiency because they can store more energy, charge faster and have a longer life than traditional lead-acid batteries. However, the technology is still in development, and fundamental progress needs to be made to meet the priority of improving the cost, range and charging time of electric vehicle batteries.
Sheng Dai, ORNL enterprise researcher and corresponding author, said: "Overcoming these challenges will require progress in more effective materials and synthesis methods that can be extended to industry“
The results published in Advanced Energy Materials showed a new type of anode material for fast charging batteries by using an expandable synthesis method. The team found a new type of molybdenum tungsten niobate compound, or MWNO, with fast charging and high efficiency, which may replace graphite in commercial batteries.
For decades, graphite has been the best material for LIB anode. In basic battery design, two solid electrodes -- positive and negative -- are connected by an electrolyte solution and a separator. In LIB, lithium ion moves back and forth between cathode and anode to store and release energy and provide power for equipment. One of the challenges of the graphite anode is that during the charging process, the electrolyte will decompose and form deposits on the anode surface. This accumulation slows down the movement of lithium ions and may limit the stability and performance of the battery.
"Because of this slow lithium ion movement, the graphite anode is regarded as an obstacle to extremely fast charging. Running Tao, ORNL postdoctoral researcher and first author, said," We are looking for new, low-cost materials that can surpass the performance of graphite. The US Department of Energy's electric vehicle extreme fast charging target is set at 15 minutes or less to compete with the refueling time of gas powered vehicles, while graphite has not reached this milestone.
"Our method focuses on non graphite materials, but these materials also have limitations. Some of the most promising materials -- niobium based oxides -- have complex synthesis methods and are not suitable for industrialization." Tao said.
The traditional method of synthesizing niobium oxide (such as MWNO) is an energy intensive process through open flame, which also generates toxic waste. A practical alternative method can promote MWNO materials to become an important candidate for advanced batteries. Researchers have turned to the mature sol-gel process known for its safety and simplicity. Different from the traditional high-temperature synthesis, the sol gel process is a low-temperature chemical method that converts liquid solutions into solid or gel materials, which is usually used to manufacture glass and ceramics.
The team transformed the mixture of ionic liquids and metal salts into porous gel and heat treated them to enhance the final properties of the material. This low energy strategy also uses ionic liquid solvents as MWNO templates that can be recovered and recycled.
"This material works at a higher voltage than graphite, and is not easy to form the so-called 'passive solid electrolyte layer', which slows down the movement of lithium ions during charging." Tao said: "Its excellent capacity and fast charging rate, coupled with scalable synthesis methods, make it an attractive candidate for battery materials in the future.
The key to the success of this material is a nano porous structure, which provides enhanced conductivity. The result provides less resistance to the movement of lithium ions and electrons, thus realizing rapid charging.
Dai said, "This research has realized a scalable synthesis method of a competitive MWNO material and provided basic insights into the future design of electrode materials for various energy storage devices.
