Single-crystal cathodes make lithium batteries more durable

A team of researchers from the Pohang University of Science and Technology (POSTECH) have demonstrated a single-crystal synthesis technology that extends the lifespan of cathode materials for electric vehicles. The research findings were published in the online edition of ACS Materials & Interfaces.

Lithium (Li) secondary batteries, commonly used in electric vehicles, store energy by converting electric energy to chemical energy and generating electricity to release chemical energy to electric energy through the movement of Li-ions between a cathode and an anode. These secondary batteries mainly use nickel (Ni) cathode materials due to their high lithium-ion storage capacity. Traditional nickel-based materials have a polycrystalline morphology composed of many tiny crystals which can undergo structural degradation during charging and discharging, thereby reducing their lifespan.

To address this issue, the cathode material can be produced in a ‘single crystal’ (a structure where all atoms are arranged in a regular manner to form one complete crystal) form. Creating nickel-based cathode materials as single large particles, or single crystals, can enhance their structural and chemical stability and durability. However, the exact process of hardening during synthesis and the specific conditions under which this occurs remain unclear.

To improve the durability of nickel cathode materials for electric vehicles, the researchers identified a single temperature, known as the ‘critical temperature’, at which high-quality single-crystal materials are synthesised. They investigated various synthesis temperatures to determine the optimal conditions for forming single crystals in synthesis of a nickel-based cathode material.

The researchers also observed the impact of temperature on the material’s capacity and long-term performance and found that conventional polycrystalline materials synthesised below a certain critical temperature are prone to degradation with prolonged use in secondary batteries. However, when synthesised above this critical temperature, high-quality single crystals can be easily produced, leading to materials with superior longevity.

This is due to a process called ‘densification’, which occurs above a certain critical temperature. During this process, the internal grain size of the material increases and the empty spaces within the materials are densely filled. Densified single crystals are hard and resistant to degradation over extended periods, significantly enhancing their durability.

Based on these findings, the researchers confirmed that synthesising single crystals above the critical temperature is a more advantageous material design strategy. They also proposed an effective method for synthesising high-quality single crystal materials.

“We have introduced a new synthesis strategy to enhance the durability of nickel-based cathode materials. We will continue our research to make secondary batteries for electric vehicles cheaper, faster and longer lasting,” said Professor Kyu-Young Park from POSTECH.

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