Elucidating Degradation mechanisms in cheap commercial olivine cathodes for Li-ion Batteries

Olivine cathodes for Li ion Batteries are extremely popular for being low-cost alternative to LCO and NMC cathodes which contain expensive elements like Ni and Co. LiFePO4 (LFP) cathodes have been widely used for EVs. LFP, however, suffers from the shortcomings of having a lower capacity and a very flat voltage profile leading to ambiguity in charge states. Improvements have been made to LFPs by doping with Mn and forming various fractions of LMFPs (LiMnxFe1-xPO4). 

These materials, although improve upon the capacity and voltage profile of LFPs, however have issues in terms of sluggish ionic kinetics. Formation of polarons due to electron-phonon coupling have been suggested as being the reason for slow kinetics of these materials. In this project we intend to investigate the possible mechanisms giving rise to polaron formation and its overall impact on these materials. Current Density Functional Theory (DFT) modelling of polarons does not involve any electron phonon coupling in these materials. 

We intend to build an Anderson-Holstein model taking into account both electron-electron as well as electron-phonon interactions. This model shall be solved within Dynamical Mean-Field Theory using the Quantum Monte Carlo (QMC) solver in the segment picture (CT-Seg). With this prescription we intend to explore the full compositional phase diagram of LMFP, with different fractions of Mn and Fe at different charge states. The main questions that we intend to investigate are origin of polarons in LMFP and its effect on different fractions of Mn/Fe, and how the polarons behave during cycling. This shall give us a detailed understanding of the redox mechanisms, and ion kinetics in LMFPs. All these studies will be linked back to experimental insights. 

This project is expected to give a complete and holistic understanding of the workings of olivine cathodes in general. The theoretical methods that will be developed while execution of the project shall be of interest to the wider many-body physics community in theoretical condensed matter research, and is expected to make faster accurate calculations including electron-electron and electron-phonon interactions possible using DMFT.

Key objectives and potential impact

• Understanding of transition metal dissolution through analysis of redox
• Understanding of impact of polaron through many body method development
• Design of futuristic and cheap olivine cathode with high conductivity, high capacity, and low transition metal dissolution
• Method and code to carry out many body polaron calculations

Any specific requirements for candidates

Computational Skills required. Basic coding in either FORTRAN, C, or Python. Basic Understanding of Solid State Physics, Quantum Mechanics, or Quantum Chemistry.

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