Experimental Investigation and Thermodynamic Modeling of LNMO High Voltage Spinel Cathode for the LIBs

• Applicant: Dr. Dajian Li
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials ; Synthesis and Properties of Functional Materials
• Term: from 2017 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 326070248

High voltage Li-Ni-Mn-O (LNMO, stoichiometric composition LiNi0.5Mn1.5O4, 4.7 V vs. Li) spinels are very promising cathode materials for Li-ion batteries (LIBs) due to the high energy densities. Consistent thermochemical and electrochemical data are urgently required for the design of power and thermal management systems of LIBs based on these new materials. Although numerous cell property investigations have been published, thermochemical (enthalpy of formation, heat capacity) and electrochemical data (entropy of reaction during charge-discharge processes) are rare. Therefore, the aim of this project is to investigate the thermochemistry and electrochemistry of LNMO spinels and to use the results to develop self-consistent analytical descriptions of the Gibbs free energy of the respective phases for the first time.The experimental studies in this proposal include the synthesis of spinel samples with selected cation ratios using the sol-gel method. Heat treatments in defined atmospheres will be used to obtain samples with different oxygen vacancy concentrations. Sample characterization will be performed using powder-XRD, ICP-OES and SEM. Redox titration techniques will be employed to determine the average oxidation state of Mn cations, allowing the calculation of oxygen vacancy concentrations. Moreover, enthalpies of formation will be measured using drop solution calorimetry, and heat capacity measurements will be conducted using different DSC devices. Battery test cells (Coin cells, Swagelok cells) will be assembled with the spinel cathode against pure Li, and will be used for potentiometric investigation to obtain the entropy changes during lithiation-delithiation process. Both LMO and LNMO spinels compositions will be investigated.A model taking the crystal structure and Wyckoff site occupancies into account (oxygen vacancy concentrations, cation mixing), will be established based on the experimental data using the Calphad approach. Lithiation-delithiation process will be simulated by modeling the relation between Gibbs energy and correct sublattice site occupations for the first time. Predictions of battery behaviors including open circuit voltages, heat generation properties, and oxygen partial pressures with temperature dependencies under varying conditions, will be performed.As the output of the current project, conditions for synthesizing LNMO spinel with improved battery performance will be proposed according to thermodynamic calculations. Meanwhile, the obtained thermodynamic models will be key contributors to the ICME development of LIBs with novel spinel cathode materials. Furthermore, the thermochemical information is essential for the battery safety and preventing hazardous phenomena such as thermal runaway and flame generation.

• DFG Programme: Research Grants