Batteries with lithium-enriched cathodes are capable of accumulating approximately 30% more energy versus their analogues with standard lithium-nickel-manganese-cobalt-oxide cathodes (NMC). However, the barrier for dissemination of such batteries is the deterioration of their performance with the growth of the number of charge/discharge cycles. The cathode material in the process of using undergoes changes, which are not fully studied yet and which may cause gradual loss of voltage and capacitance.
Scientists agree that the problem is linked to the oxidation-reduction processes engaging oxygen. However, the essence of this link to oxygen atoms is not understood, and this theoretical gap impedes finding solution to the problem.
One of the commonly accepted (until recently) hypothesis was as follows: during the service life of a battery oxygen atoms initially built into the cathode crystal lattice are gradually released and generate O2 molecules in the pores of the material. The molecular oxygen displays almost zero electrochemical reactivity, hence, its accumulation worsens the battery performance. Identifying O2 molecules in the lithium-enriched cathode materials with the help of X-ray spectroscopy spoke in favor of this hypothesis. However, it cast doubt on using any technologies, because molecular oxygen generation is an irreversible process, and it is difficult to affect it.
Scientists from Skoltech, Montpellier University and College de France succeeded in dismissing this hypothesis based on the results of a series of tests performed in the European Synchrotron Radiation Facility (ESRF). The authors learned that the binary molecule of gaseous oxygen (O₂), which was earlier found in the cathode of lithium-ion battery was generated due to exposure of X-rays along the X-ray spectrum (RIXS).
“Fortunately, after publication of our articles we can state the hypothesis about the molecular oxygen in the pores of cathode material can be recognized dead and gone. We analyzed the data obtained in the course of extensive X-ray scattering tests, and demonstrated that oxygen molecules blocked in the cathode — which we believed to be the cause of worsening the performance — in reality with high probability are measurements artefacts. It means, they were generated under the X-rays exposure used to find them”, Skoltech is citing Dmitry Aksyonov, an employee of the Energy Technologies Center.
This discovery allows for significant decrease of the uncertainty, which for a long time existed around the mechanism of oxygen oxidation in cathodes. Now the realm of the research in the sphere of cathode material stabilization is narrowed down to the so-called structured oxygen – atoms losing one electron in the process of charging the battery. It will be much easier to cope with this task compared to molecular oxygen in the pores. This makes commercialization of new batteries more realistic.
