The quest for sustainable energy resources has led to the development of a novel entity of redox active polyanionic compounds which have the capability to deliver high energy densities, which is essential for electrochemical energy storage. Even better, the robust framework and maintaining the stability of the material in a wide electrochemical stability range causes an enhanced interest to implement these polyanionic compounds as cathodes for rechargeable alkali ion batteries, hybrid super capacitors and materials for the oxygen evolution reaction. Olivine cathode materials have already been identified as potential candidates for application in electric vehicles. Unfortunately, the extremely low electronic conductivity and slow lithium ion diffusion rate strongly limit the power density of the olivine materials. Tailoring of olivine structure with polyphosphates could essentially improve electrochemical activity of the olivine-based cathode material without using conductive carbon. However, the charge transfer mechanism triggered by lithium ion removal, as well as the stability of the tailored material consisting of different crystallographic phases are still unknown.
Recently Technische Universität Darmstadt researchers in Germany: Dr. Gennady Cherkashinin and colleagues investigated the stability of the novel highly conductive 5 V cathode material, consisting of olivine-LiCoPO4 (LCP) and lithium dicobalt tripolyphosphate (LCPO), under electrochemical delithiation. The evolution of the electronic structure, oxidation state and spin state were in-depth studied by employing quasi in-situ depth-resolved synchrotron photoelectron spectroscopy, X-ray absorption spectroscopy, whereas changes in the crystallographic structure as a function of the charging potential were studied by X-ray diffraction. Their work is currently published in Journal of Material Chemistry A.
The authors observed that withdrawal of a Li+ ion from the cathode upon charging leads to release of a valence electron from the Co3d state, whereas the O2p state was not involved in the charge compensation until reaching a deep stage of delithiation, which corresponds to 5.1 V vs. Li+/Li. Additionally, the authors noted that the local electronic structure at the oxygen site and phosphorus site was correlatively affected in its fully delithiated state, which was assigned with a change in the Co3d–O2p hybridization, as well as altering of the polarization in Li–O–P bonding due to Li+ removal. Further, the novel material obeyed a rigid band behavior of the electronic structure, which in fact correlated with the stable crystal structure in its fully delithiated state thereby suggesting a high intrinsic stability of the tailored material upon delithiation/lithiation. The authors have also revealed that the delithiated LCP-LCPO tailored material was not amorphized after exposure to air for several hours, which is the advantage compared to the conventional LCP. The capacity loss observed via electrochemical cycling might be associated with the dissolution of the secondary phase of the olivine-based material contacting to the electrolyte. Thus, coating of LCP–LCPO with a protective layer, as well as the formation of a stable solid electrolyte interface under a high working potential could improve the cycling stability of the battery cell for long-term performance.
Gennady Cherkashinin, Mikhail V. Lebedev, Sankaramangalam U. Sharath, Andreas Hajduk, Silvia Nappini, Elena Magnano. Exploring redox activity in a LiCoPO4–LiCo2P3O10 tailored positive electrode for 5 V lithium ion batteries rigid band behavior of the electronic structure and stability of the delithiated phase. Journal of Material Chemistry A, 2018, volume 6, page 4966.Go To Journal of Material Chemistry A