(Invited) Investigation of Oxygen Release from Cathode Active Materials for Lithium-Ion Batteries Based on Anion Defect Chemistry

Ensuring safety and reliability of the battery is crucially important for further widespread of electric vehicles. For that purpose, understanding lattice oxygen stability of cathode active materials is important fundamental knowledge, because exothermic reaction between flammable organic liquid electrolyte and oxygen come out from cathode active material triggers thermal runaway. Operando/in-situ measurements and simulations revealed oxygen release of de-lithiated cathodes by thermal decomposition and heat generation in a commercial battery [1-3]. Although these works revealed exothermic reaction in a real battery cell, inherent lattice oxygen stability of cathode materials is not understood well. In this work, we investigate oxygen release behavior and lattice oxygen stability of Li(Ni,Co,Mn)O2 (NCM) to demonstrate the effectiveness of defect chemical and thermodynamic evaluations. NCM powders were synthesized by solid-state reaction from Li(OH)·H2O and Ni-Co-Mn hydroxide precursors obtained by co-precipitation. Equilibrium oxygen release behavior was evaluated by coulometric titration and thermogravimetry. The electrochemical cell for coulometric titration was fabricated from Yttria stabilized Zirconia (YSZ) tube with Au current collectors. The specimen was placed in the YSZ tube and sealed. Titration was carried out at 673-873 K. Reduction behavior due to oxygen release was investigated by X-ray absorption spectroscopy. Equilibrium relation between oxygen content as functions of temperature and PO2 was obtained by coulometric titration and thermogravimetry. With increasing Ni, NCMs release lattice oxygen easily. To quantitatively confirm this tendency of stability, partial molar enthalpy of oxygen which represents necessary energy for oxygen release is estimated based on thermodynamic analysis. It was found that high-valent Ni destabilize lattice oxygen drastically. When partial molar enthalpy of oxygen is plotted as a function of the amount of released oxygen, two groups were clearly classified. One is oxygen release by reduction of high-valent Ni which requires about 0.5 eV, and the other is that by reduction of Co which requires more than 2 eV. This tendency strongly suggests that lattice oxygen stability is essentially determined by reduction species and high-valent Ni deteriorate the stability of lattice oxygen drastically. In the presentation, detailed experimental procedures and experimental results are going to be shown and discussed [4, 5]. In addition, recent work on anion defect chemistry of cathode active material will be shown in the presentation. Reference [1] D. Finega, Nat. Commun., 2015, 6, 6924. [2] S. Bak, Chem. Mater., 2013, 25, 337. [3] Q. Wang, J. Power Sources, 2012, 208, 210. [4] X. Hou, Adv. Energy Mater., 2021, 11, 2101005. [5] X. Hou, ACS Energy Lett., 2022, 7, 1687. Acknowledgement This work is supported by KAKENHI grant No. 24H02205, ASAHI Glass Foundation and JRP-LEAD with DFG.