Lithium-ion batteries involve an intricately designed chemical recipe, one that scientists are constantly trying to improve. The latest tweak from researchers at Pacific Northwest National Laboratory and their collaborators involves a novel way of synthesizing cathode materials, adding a step that could help batteries last longer and perform better. A paper capturing the details recently appeared in the journal Nature Energy.
The work involves fortifying the boundary between a particular type of cathode and the liquid electrolyte. The cathode material, known as nickel-rich NMC (nickel-manganese-cobalt), "is one of the most energy-efficient cathode materials used worldwide in the last 10 or 20 years," said Ji-Guang Zhang, a lab fellow in PNNL's Energy Processes & Materials Division who co-authored the paper.
But two problems begin to surface with nickel-rich NMC once the battery is in use. First, particles within the cathode begin to disintegrate after many charge and discharge cycles; the particles also begin to react with the liquid electrolyte, which is worsened by the disintegration. Both issues eventually cause the battery's voltage and longevity to fade.
To stave off these reactions, a PNNL team, working together with Xueliang Sun of the University of Western Ontario in Canada, coated lithium phosphate (LPO) to the surface of cathode using atomic layer deposition (ALD), a technique for creating thin films that results in a two-dimensional coating, a bit like fondant over a cake.
This method had been tried before, but the surface coating has a limited effect on cathode protection. The PNNL team went further, heating the coated cathode to 600 °C (1112 °F) in air for a few hours. With this step, the ALD layer infused into the inside boundaries of the particles.
“Think of a cake, topped with hot chocolate that soaks into the top layer instead of fondant,” said Chongmin Wang, a microscopic characterization expert on the PNNL research team. “This three-dimensional infused layer bonded more strongly with the cathode, stabilizing its boundary and preventing particles from cracking.”
The researchers found that a cathode infused with the LPO electrolyte retained 91.6 percent of its capacity after 200 charging cycles, versus 79 percent for an untreated cathode.
The finding opens up new possible designs for advanced battery cathodes needed for electric cars. "This method can be applied not only to the material we are using in the paper,” Zhang said, “but also for other cathode materials.”
This research was supported by DOE’s Office of Vehicle Technologies.