Materials called NMCs are widely used as the positively charged electrodes – or ‘cathodes’ – in lithium-ion batteries, making them key components in everyday devices ranging from smartphones to electric cars. One type of nickel-rich NMC, called NMC
955, is currently being developed as a promising new cathode material, owing to its exceptionally high energy density. Although the high nickel content of NMC
955 helps reduce the need for cobalt, improving its environmental impact, it also causes structural and thermal stability issues that lead to safety concerns and reduced electrochemical performance. As a result, batteries with this cathode material can lose up to 15% of their charge capacity after just one charge-discharge cycle.
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Within the Horizon Europe PULSELiON project, Helena Braga’s group and their colleagues have demonstrated how carefully choosing materials and synthesis methods can overcome these challenges.
The team began their process by replacing the flammable liquid electrolytes in traditional NMC batteries with a sulfur-based solid one, which is still highly capable of transporting lithium ions between the anode and cathode. Secondly, they incorporated an alternative to the electrode’s binder: used to hold its constituent materials together. Instead of the fluorinated, environmentally damaging material that is normally used, they applied a sustainable, rubber-based alternative.
To combine these components, the PULSELiON team developed a ‘wet mixing’ method, where the binder is first dissolved in toluene to create an even solution. They then combined this solution with the solid electrolyte and NMC955 cathode material within a sealed, inert container. This prevented any moisture or oxygen from reacting with the mixture, which could degrade the battery materials in the future.
Still inside the inert container, the PULSELiON team then spread the mixture onto a thin aluminium substrate to dry, ready to be cut into disks. To assemble the battery, they sandwiched a lithium anode, the solid electrolyte and one of these cathode disks – with each component fitting tightly together.
Finally, the team used first-principles computer simulations to identify the most effective voltage range over which the battery should be charged and discharged. Impressively, this allowed the researchers to predict the charging and discharging curve with great accuracy. This showed them the best charge-discharge cycling profile to maintain strong performance, while avoiding damage and oxygen release.
By testing the battery, the PULSELiON team confirmed that it retained its charge-carrying capacity for well over 200 charge-discharge cycles, without any need for applied heat or high pressure. They hope that their new concept for applying NMC955 in lithium-ion batteries could help our devices to perform better, while also greatly improving their safety and sustainability.
