New highly performing Ni-rich cathode active materials for all-solid-state batteries

To support the further advancement of the electronics industry, energy researchers have been trying to develop new battery technologies that could be charged faster, power devices for a greater amount of time and have longer overall lifetimes. Some of the most promising emerging batteries that could meet these requirements are all-solid-state batteries (ASSBs).

ASSBs are batteries that contain solid electrolytes, as opposed to the liquid electrolytes found in conventional battery technologies. Compared to lithium-ion (Li-ion) batteries, which are currently the most widely used rechargeable batteries, ASSBs could be safer, as solid electrolytes are typically less likely to catch fire, and could also store more energy (i.e., have higher energy densities).

A central component of these batteries is the so-called cathode active material (CAM), a component that stores and releases lithium ions. Layered materials rich in nickel (Ni) have been particularly promising CAMs, yet they were also found to exhibit significant limitations.

Specifically, past studies showed that these cathodes can reduce the ability of ASSBs to hold charge over time, a process known as capacity fading. The reduction in capacity they prompt was linked to chemical reactions at the interface between Ni-rich cathodes and electrolytes, as well as the expansion, contraction and disintegration of cathode particles.

Researchers at Hanyang University in South Korea recently carried out a study to better understand how the amount of Ni in CAMs influences the degradation of ASSBs. Their findings, published in Nature Energy, informed the development of new Ni-rich cathodes that could boost the performance and lifetime of ASSBs.

"ASSBs comprising Ni-rich layered cathode active materials (CAMs) and sulfide solid electrolytes are promising candidates for next-generation batteries with high energy densities and safety," wrote Nam-Yung Park, Han-Uk Lee and their colleagues in their paper. "However, severe capacity fading occurs due to surface degradation at the CAM–electrolyte interface and severe lattice volume changes in the CAM, resulting in inner-particle isolation and detachment of the CAM from the electrolyte."

As part of their study, Park, Lee and his colleagues first set out to identify each factor influencing the degradation of ASSBs with Ni-rich CAMs and quantify their effects. To do this, they synthesized four different types of Ni-rich cathodes, with Ni content ranging from 80 to 95%.

These included pristine Li[Ni_xCo_yAl_1−x−y]O₂ cathode materials, boron-coated CAMs, Nb-doped CAMs and CAMS that were both boron-coated and Nb-doped CAMs. They then closely examined how these different cathode materials and their Ni concentrations influenced the degradation of ASSBs.

"We quantified the capacity fading factors of Ni-rich Li[Ni_xCo_yAl_1−x−y]O₂ composite ASSB cathodes as functions of Ni content," wrote Park, Lee and their colleagues. "Surface degradation at the CAM–electrolyte interface was found to be the main cause of capacity fading in a CAM with 80% Ni content, whereas inner-particle isolation and detachment of the CAM from the electrolyte play a substantial role as the Ni content increases to 85% or more."

Overall, the researchers found that surface degradation at the interface between the Ni-rich cathode materials and electrolyte was the primary cause for the degradation of ASSB capacity. The isolation of inner-particles and their detachment from the cathode materials, on the other hand, was found to only affect the capacity of batteries when the cathodes contained over 85% of Ni.

Park, Lee and his colleagues subsequently drew from their findings to develop new Ni-rich CAMs with an altered surface and morphology. These materials have columnar structures that were found to effectively reduce the detachment of particles from the cathode materials and inner-particle isolation.

When deployed in a pouch-type full cell with a C/Ag anode-less electrode, these new cathodes retained 80.2% of their initial capacity after 300 operation cycles. The valuable insight gathered by this research team and the cathodes they developed could soon help to improve the performance of ASSBs further, potentially contributing to their future deployment and widespread adoption.

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