In a paper published in the journal Energy Storage Materials, the scientists explain that the limited, fraught supply chain of cobalt creates a bottleneck for large-scale battery production, including for manufacturing the ones used in electric vehicles. In addition to this, cobalt extraction generates toxic waste.
To address these issues, lithium nickel oxide (LiNiO2) — which is similar in structure to the widely used lithium cobalt oxide (LiCoO2) — often serves as a cobalt-free alternative for electrode material. However, key instability issues plague the compound, specifically a gradual loss of capacity at the high-voltage region, which is associated with nickel-ion migration.
To improve electrode reversibility, nickel ions have been partially substituted by other metal ions, including reintroduced cobalt ions as well as manganese, aluminum and magnesium. This creates “nickel-enriched layered materials” to serve as positive electrode materials.
“So far, 10-20% cobalt ions were necessary for nickel-based electrode materials,” Naoaki Yabuuchi, senior author of the study and a researcher at Yokohama National University, said in a media statement.
In Yabuuchi’s view, such a percentage is still too high and is the result of not having established a unified understanding of how metal substitution can improve the process.
A migration issue
To address this knowledge gap, he and his collaborators dug deeper into the problematic phase transition. When lithium ions leave the cathode under the influence of an external field, nickel ions migrate to specific sites within the lithium layers. Although this process is reversible, the reversibility gradually degrades through continuous cycles until the capacity is completely lost — a phenomenon not seen in cobalt-ion migration.
Previous studies reported that tungsten doping in LiNiO2 is an efficient approach to suppressing the detrimental phase transitions at high-voltage regions. Thus, Yabuuchi’s group tested the hypothesis that heavy, expensive tungsten ions could be substituted with other elements, specifically phosphorous — a lighter, more abundant element.
After a detailed analysis of LiNiO2 integrated with nanosized lithium phosphate (Li3PO4), the researchers observed that, under certain conditions, problematic nickel-ion migration was effectively suppressed due to repulsive electrostatic interaction from the extra nickel ions within the Li layers. Moreover, from these findings, Li-deficient LiNiO2, Li0.975Ni1.025O2, with the extra nickel ions in Li layers, was also synthesized using a simple methodology without phosphorus integration.
The results also showed how Li0.975Ni1.025O2 can effectively mitigate unfavourable nickel-ion migration, and deliver consistent reversibility without cobalt ions.
“These findings open a new direction to develop high-performance and practical cobalt-free nickel-based electrode materials with an extremely simple and cost-effective methodology,” Yabuuchi said. “This material achieved the ultimate goal for high-performance nickel-based electrode materials.”
In future endeavours, the researchers plan to investigate the feasibility of a nickel-free material to support lithium-ion batteries.