Recent Achievements on High Voltage Olivine-Type Cathode Materials
Authors: Cherkashinin Gennady , Sharath S U , Jaegermann Wolfram 
Published: February 2020

The development of high power and high energy density cathode materials for rechargeable Li-ion batteries suitable with high operation voltage is a challenge. In this regard, the fundamental understanding of the thermodynamic stability and performance limits of high voltage cathode materials is essential, since one allows us to predict/determine the intrinsic voltage limit of the cathode material [1, 2] and tailor the components of the battery cell to provide a high Li+ diffusion rate in the bulk material and across the interface. The main reasons which limit to use high voltage LiMPO4 (LMPO, M=Co, Ni) cathode materials for electric vehicle applications are low electronic conductivity, which results from the olivine structure where the valence electrons of the transition metal are isolated from each other by the polyanion (PO3-
4), and an instability of the electronic structure of LMPO at the electrode-electrolyte interface caused by the decomposition of liquid electrolytes at high voltage potential. Low electronic conductivity of the Co- and Ni based olivine-type cathode materials makes it difficult to use electronic spectroscopy techniques which give unambiguous information on the density of the occupied and unoccupied states near the Fermi level (E
F), on the oxidation and spin states of the transition metals, as well as on the chemical composition at the electrode-electrolyte interface [3-5]. In this presentation, we demonstrate our latest achievements on the study of the electronic structure of the layered- and olivine- type film cathode materials and its correlation with the electrochemical performance, Fig. 1. We demonstrate that the electronic structure of the bulk cathode material and the cathode-electrolyte interface is a key parameter which governs the thermodynamic stability of the whole battery cell. By plotting the experimentally obtained energy diagrams of the LiCoO2 (LCO) layered structure and LCPO-olivine structure cathode materials, we demonstrate the role of the inductive effect [7] which leads to the increase of the voltage window by the substitution of oxygen (O2-) by the polyanion. We discuss the role of morphology of the LiMPO4 film cathode materials on the Li-ion rate and demonstrate that high electronic conductivity can be achieved without using the carbon additives to the active material, Figs. 1(b). On the example of a LCPO film with different lithium content, x, we demonstrates the gradual shift of E
F to the valence band maximum due to the increase of the work function of the cathode material in accordance with the rigid band model. By the analysis of the evolution of the core-levels and valence band structure as a function of lithium content in LCPO, we demonstrate that the charge compensation caused by Li+ removal takes place at the Co site resulting in the change of Co2+ ions (x=1.0) with the high spin (HS) state to the Co3+state in the accordance with DFT calculations [8]. The possible physicochemical mechanisms contributed to the degradation of the LMPO batteries are also discussed.
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Figure 1