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Cathode coating for longer battery life and greater safety

Surface coating of cathode materials has been widely investigated with a view to enhance the life and rate capability of lithium-ion batteries, but computationally identifying materials with suitable ionic conductivity can be a very challenging task: creating a more efficient cathode involves addressing a myriad of factors simultaneously, from keeping the cathode of a battery electrically and ionically conductive to making sure that the battery stays safe after many cycles.

Now (2020), scientists at the Argonne National Laboratory in collaboration with Hong Kong University of Science and Technology (HKUST) have designed a new particle-level cathode coating for lithium-ion batteries which is able to increase their life and safety. The researchers were studying cathodes made up of metal oxides consisting of nickel, manganese and cobalt. The new coating was created out of a conducting polymer called poly(3,4-ethylenedioxythiophene) (PEDOT), which is a completely new approach in lithium-ion battery technology since it completely protects each particle of the cathode on the inside and outside from reacting with the electrolyte. PEDOT was applied by means of Argonne’s oxidative chemical vapour deposition technique, which used gas to ensure the coating reaches every particle of the cathode and formed a robust skin. Researchers used an ion beam-scanning electron microscopy dual-beam system and an energy-dispersive X-ray spectrometer to make sure the coating of PEDOT on both primary and secondary particles of layered cathodes had worked and that they were stable after battery cycling.

Scientists have long sought to improve battery life using better coating techniques. In 2017, scientists studied the effect of the composition of Ni-rich LiNixMnyCo1-x-yO2 (NMC) on the surface alumina coatings. Changing cathode composition from LiNi0.5Mn0.3Co0.2O2 (NMC532) to LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811) increased the diffusion of the surface alumina into the bulk after high-temperature annealing. Employing a variety of spectroscopic techniques, aluminium was observed to have a high bulk compatibility with higher Ni/Co content, whereas low bulk compatibility was associated with Mn in the transition metal layer. The scientists also found that the cathode composition affected the observed morphology and surface chemistry of the coated material, which had an effect on electrochemical cycling. The presence of a high surface Li concentration and strong alumina diffusion into the bulk led to a smoother surface coating on NMC811 without aggregation of excess alumina on the surface.

In 2019, scientists introduced LiInO2 and LiInO2–LiI as new cathode coating materials. The LiInO2–LiI composite coating layer reduced undesirable interfacial reactions and inhibited the diffusion of S and P ions from the sulphide electrolyte to the oxide cathode. Also, the electrochemical properties of all-solid-state cells were enhanced by the cathode coating. The LiInO2–LiI-coated electrode created better rate capability and lower impedance than the LiInO2-coated electrodes. The research showed LiInO2–LiI composite coating was successful at improving the cathode stability while providing superior electrochemical properties.

The main advantage of the new cathode coating is that it can help improve the battery voltage to 4.6V, as the coating helps to transport lithium ions and electrons in and out of the cathode more efficiently. Currently, lithium-ion batteries operate at 4.2 V at the cell level. This 15 percent difference can lead to a significant cost reduction of the overall battery pack. Also, the new cathode material can prevent the formation of an unwanted film on the cathode under high temperatures, which oxidises the electrolyte and causes energy loss.

There is no doubt that the new battery cathode has the potential to change the way we live. It can help increase the driving range of electric cars and increase the battery life of cell phones and laptops, which will ultimately contribute to more sustainable energy usage.