(Invited) Bi-Layer Cathodes Comprising Different Active Material Sublayers Demonstrate Superior Fast Charge Capability

Abstract

Sluggish electrolyte transport properties result in a tradeoff between energy density and rate capability in lithium-ion batteries. To increase energy density, electrodes are made thicker and less porous. However, once thick enough, lithium transport in the electrolyte becomes the rate limiting process, and capacities at elevated C rates are reduced as a result of underutilisation of active material near the current collector. Strategies have been proposed to overcome these limitations, including pore engineering to reduce through-plane tortuosity, to varying degrees of success.We introduce bilayer cathodes that aim to improve the rate performance of thick electrodes by controlling the through-thickness charging rate. The bilayer cathode structure is comprised of two discrete sublayers containing the active materials lithium iron phosphate (LFP) and lithium nickel manganese cobalt oxide (NMC). Due to LFP and NMC having open-circuit voltage (OCV) profiles in different voltage windows, the through-thickness charging rate is dependent on the location of the two active materials. The bilayer electrodes are manufactured by multi-pass doctor blade coating and, in principle, could also be produced using twin-slot dies which would require only a minor modification to the current commercial manufacturing methods. The electrochemical performance of the bilayer cathodes are compared with a blended electrode (single layer containing intimately mixed LFP and NMC) and a uniform NMC electrode, all at constant areal capacity (4.5 mAh cm-2) and porosity (30%). We report significant differences in voltage profiles when charging from 0% state of charge as well as intermediate (e.g. 50%) states of charge in electrodes containing the same mass fraction of LFP and NMC, demonstrating that the location of the active material through the electrode thickness impacts behaviour. Moreover, the best performing bilayer electrode structure (LFP layer adjacent to the current collector, NMC layer on top of the LFP layer) outperforms the uniform NMC electrode in fast charge tests. At 3C, the best performing bilayer cathode maintains 84% of its capacity whilst the uniform NMC electrode maintains only 53%. In discharge, the same electrodes both maintain approximately 52% capacity at 3C, demonstrating the anisotropic charging/discharge performance introduced by the bilayer structure.An understanding of the through-thickness charging rate, and distribution of state of charge, throughout charging is required to explain why the bilayer cathode outperforms a conventional uniform cathode. In uniform electrodes, with the same active material through the thickness of the electrode, a gradient in state of charge is formed due to a gradient in resistance to charge through the electrode thickness. This is due to much higher ionic resistance within the electrolyte than the electronic resistance within the composite electrode. In thick electrodes this effect is pronounced enough so that at the end of charge there is far greater underutilisation of active material in the part of the electrode nearest the current collector. In the bilayer cathode structure, by placing active material with a lower OCV near the current collector, we can, somewhat counterintuitively, achieve much more even through-thickness charging compared to uniform electrodes, minimising underutilisation of active material near the current collector and increasing rate performance.