Cathodes for Lithium-ion Batteries: Understanding the materials that will take you further

Professor Jose Alarco, QUT

Together with the anode, separator and electrolyte, the cathode is a major key component of a battery cell that contributes to the capacity and delivery of the stored electrochemical energy.

A rough estimate of the energy in a battery cell is given by the product of the average voltage and current during operation of the battery. Most applications require or desire the highest energy stored in the smallest volume with the lightest weight, while remaining safe, and at the lowest possible cost. The smallest volume and weight for a given high energy make the battery more portable and efficient. The delivery of the charge also needs to be at a reasonably high rate, otherwise a low flow rate of current would require a much larger total capacity to produce the necessary total flow. We can think of the size of a bucket of water as the total capacity. If the flow is low per bucket, just like dripping, we would require more buckets to combine their drippings into a measurable and needed total flow of water.

The cost is largely determined by the choice of raw materials, the crystal structures and their manufacturing processes. Australia is rich in the minerals, which become the raw materials for mixed Lithium (Li), Nickel (Ni), Cobalt (Co) and Manganese (Mn) precursors, known in acronym form as NCM materials. These materials have become a focus for the growth of a battery industry in Australia, with support of industry and government through programs such as the Future Battery Industry Cooperative Research Centre (FBI CRC) and storEnergy.

Schematic of the crystal structure of NCM materials. Purple, red and blue balls represent mainly Lithium atoms, Oxygen atoms and the Ni-Co-Mn atomic combination, respectively.

The transition metals, Ni-Co-Mn, are typically components in a series of catalyst formulations. That is, they can be very reactive, particularly if they are present in very small particle size. These characteristics place constraints on the desirable size range, agglomeration, particle surface characteristics, etc. of the cathode material, which will determine the performance and the interaction with the electrolyte. Cyclability is another performance criterion, which determines how much capacity is retained per cycle over many years. Batteries are typically replaced once the capacity drops below 80% of the original value. At that point the total cost of the battery has to be reinvested, which preferably should not happen too soon, at least not under 5 or 7 years, preferably 10.

Industry has opted for a co-precipitation approach in the manufacture of NCM materials. This choice gives the best economy of manufacture, while retaining the ability to adequately scale up, and gives good control of particle characteristics in terms of individual particles and agglomerate sizes. While size control of the particles and quality of agglomeration contributes to the durability and reproducibility of properties, the interaction of the cathode with the electrolyte still needs to be moderated by a tailored surface coating. This will be one of the biggest determinant factors on the NMC cathode long-term cyclability.

The other major aspect of NCM materials preparation are the ratios of the Ni, Co, and Mn. The original concept was developed using a 1:1:1 molar ratio of metals, however it is strongly desired that the formulations head towards lower cobalt and high nickel compositions. This is because the cobalt is expensive – far too expensive and difficult to supply for a growing EV market – while the increase in Ni increases the operating voltage (and thus the energy density). Currently NCM811 (80% Ni, 10% Co, 10% Mn) compositions have been deployed for mass market applications, while high Ni, low Co compositions of around 90:5:5 are under development. Issues around handling and stability in air/moisture, as well as reactivity with the electrolyte still need to be addressed. Post-processing and coating of the particles is used to help achieve this, along with electrolyte additives.

Our group at QUT, has had several years of experience with manufacture of LiFePO4 (LFP) cathode materials at pilot scale and full microstructural and electrochemical characterisation of these materials. Phosphates are materials which display exceptional stability; therefore, families of phosphates offer great opportunities for desirable coating materials for NCM cathodes. Our most recent cathode research is focused on NCM materials made by co-precipitation approaches at pilot scale and the development and optimisation of coatings for long-term cyclability. This work also involves understanding of interface phenomena. We have been fortunate to obtain awarded beam time access at the Australian Synchrotron, which greatly complements our related fundamental investigations.

Finally, a high energy density cathode alone does not make the best showcase of a high energy density battery. The high energy of a cathode, if matched to a low energy density anode, requires such a large volume of anode that the limited, fixed volume of the cell is taken up quickly. To get the most out of the battery, a matched combination of both high energy density cathode and anodes is required. In that way thinner coated electrodes can be fabricated, and more turns rolled inside a cell, for a cylindrical 18650 (18 mm diameter by 65 mm high) format. One of our PhD students is proudly working on high energy density anodes with the support of Stor Energy. These anodes when optimised will be matched to the optimised NCM outputs, and subsequent activities will explore incorporation of suitable additional components developed by collaborators around Australia, worldwide or in-house.