Boron takes the strain out of Li-ion battery electrodes

Widespread adoption of electric vehicles (EVs) is needed to reduce carbon emissions and dependence on fossil fuels, but limited range and high cost puts off many would-be buyers. Despite improvements, Li-ion batteries powering modern EVs are restricted by the driving range per charge, which depends on the capacity of the cathode.

“Improved Li-ion batteries capable of providing higher energy and power density and longer service life are much sought after for the commercial success of EVs,” says Yang-Kook Sun of Hanyang University in Korea.

Together with colleagues at Lawrence Berkeley National Laboratory and Forschungszentrum Jülich, Sun has developed Ni-rich layered LiMO₂ cathodes that can simultaneously deliver high energy density and a long battery lifetime [Ryu et al., Materials Today (2020), https://doi.org/10.1016/j.mattod.2020.01.019].

“Generally, the intrinsic trade-off relationship between capacity (driving range) and cycling stability (battery lifetime) is observed for layered cathodes, which have become standard cathodes for EV Li-ion batteries,” he points out.

Currently, Ni-rich lithium nickel cobalt aluminum oxide (NCA) cathodes used in EVs, such as Tesla models S, X and 3, are only partially discharged (to around 60%) in each cycle to maintain stability. Not only does this reduce energy density, it also adds to the deadweight of the battery, increasing the overall cost of EVs. In the deeply charged state, microcracks can form that, if allowed to propagate to the surface, allow electrolyte to seep in, resulting in unwanted or ‘parasitic’ reactions, which degrade internal surfaces ultimately leading to failure.

Sun and his colleagues have found that tailoring the microstructure of the cathode can solve the problem. Adding small quantities of boron to Ni-rich NCA changes the microstructure dramatically from spherical particles to elongated rod-like structures radiating out from the center. When subject to repeated cycles of charging, the crystals show little evidence of microcracks and those that do appear do not reach the surface. The result is boron-doped cathodes that retain over 80% of their initial capacity after 1000 cycles compared with undoped NCA, which retains only 49% of its initial capacity.

“Many strategies have been tried to overcome the shortcomings of Ni-rich layered cathodes, but most strategies are focused on simple doping and coating,” points out Sun. “We approached the issue from a different angle. To improve the structural and mechanical stabilities of cathode materials, we modified the particle microstructure such as the particle size, shape, and crystallographic texture.”

Doping NCA cathodes with boron could, if combined with other stabilization strategies such as protective coatings, provide the boost to energy density and stability needed to push the driving range per charge of EVs beyond the 300-mile threshold, the researchers believe.