Lab-Scale Operando X-Ray Emission and Absorption Spectroscopy to Monitor Redox Processes of Transition Metals in Lithium-Ion Batteries

Tracking changes in the electronic structure of transition metal during cycling of lithium-ion batteries (LIBs) is crucial to investigate the nature of redox reactions occurring at the electrodes. X-ray absorption spectroscopy (XAS) is often utilized to study the role of individual elements in the redox processes during the cycling of batteries through oxidation state information in real-time (operando). In this study, X-ray emission spectroscopy (XES), a complementary core-hole method to XAS is presented as an alternate method to study the characteristic emissions from battery to investigate chemical and spin state changes in electrode material during C/10 constant current cycling.<br/><br/>XAS measures the energy-dependent absorption coefficient of a material in the process of generating core electron holes. Chemical state sensitivity from the XAS near-edge structure (XANES) and local atomic structure sensitivity through extended x-ray absorption fine structure (EXAFS) allows us to keep track of changes in the constituent elements during cycling. XES, a complementary method is obtained by capturing characteristic photons resulting from quenching generated core holes. Kβ emission line (3p-to-1s transition) of XES obtain their chemical state sensitivity through interaction between the 3p and valence orbital (exchange interactions). Element-specific emissions of sufficient energy resolution are simple to process and inform us on the chemical state, spin state, and local structure of transition metal allowing us to study the redox processes in real-time (operando) during charge/discharge of batteries. The high flux requirement of such real-time (operando) experiments often limits these studies to synchrotron sources. Limited access to synchrotron radiation has hastened the development of lab-scale instruments that employ bremsstrahlung sources and monochromators with sufficient energy resolution. The lab-scale instrument used in this study can perform both XAS and high energy-resolution XES experiments using a scanning setup which employs the Rowland circle geometry.<br/><br/>Here, we present the spectral changes of transition metal Kβ_1,3 and Kβ’ features of XES are utilized to track state of charge (SoC) and study redox processes during C/10 operando cycling for the following cathode materials: LiCoO₂ (LCO), Li[Ni_1/3Co_1/3Mn_1/3]O₂ (NMC111), Li[Ni_0.8Co_0.1Mn_0.1]O₂ (NMC811), and LiFePO₄ (LFP). The results obtained from operando Kβ XES are compared with K-edge XAS, which serves as a benchmark. Additionally, the sensitivity of XES towards the total spin of the transition metal valence is used to investigate the change in magnetic properties resulting from nucleation of a new phase in electrochemically delithiated LCO samples in the range of 2-10% lithium removal.