A FIRST PRINCIPLES STUDY ON INTERCALATION VOLTAGE AND REDOX MECHANISM FOR MONOCLINIC NA3V2(PO4)3 AS SODIUM ION BATTERY CATHODE MATERIAL

• Main Author: Pradipta Wijaya, Abel
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/85399
• Institution: Institut Teknologi Bandung

Sodium ion battery (SIB) is a promising alternative as energy storage system (ESS) due to its raw material abundance compared to lithium ion battery (LIB). However, the electrochemical performance of SIB is still not comparable to LIB due to larger ionic radius that hinders in the intercalation process when directly using electrodes that work with LIBs. Na3V2(PO4)3 or better known as NVP, is a promising SIB cathode material with its 3D framework NASICON structure giving it good structural stability and fast sodium ion conduction network. The majority of earlier experimental studies shows that NVP exhibit rhombohedral R3-c symmetry. However, one study conducted by Chotard in 2015 reveals that NVP appears to undergo a monoclinic C2/c distortion below ambient temperatures. The physical and electrochemical properties of this NVP crystal phase is limited. To achieve better understanding of NVP as SIB cathode material, further research needs to be conducted. In the field of battery research, the physical properties of materials is used to study intercalation voltage and the redox mechanism among others. This research is done with a first principles or ab-initio approach based on density functional theory or DFT. A generalized gradients approximation (GGA) based exchange-correlation functional is used to account for electronic contributions in the studied material. In our research, the GGA developed by Perdew-Burke- Ernzerhof (PBE) is used. As battery cathodes are solid state materials, we also opted to use revised PBE for solids (PBEsol) and examine the results between the two exchange-correlation functional. The intercalation voltage of monoclinic NVP shows similar values between the two exchange-correlation functionals, with 1,89 – 2,64 V using PBE and 1,84 – 2,65 V using PBEsol. These values are higher than 1,41 – 2,35 V, which is the voltage of the rhombohedral NVP counterpart done by our fellow research colaborator. The predicted voltage using only PBE and PBEsol is still lower than reported experimental findings of 3,30 – 3,40 V. It is crucial to delve into the electronic structure or redox mechanism of NVP, particularly of vanadium. The redox mechanism of NVP can be studied from vanadium (V) magnetization moment and its bond length with oxygen throughout different concentrations of sodium ion in NaxVP. The results using PBE and PBEsol also give similar values. The magnetization moment of V are 2,2 ?B in Na4VP, 1,8 ?B in Na3VP, 1,4 ?B in Na2VP, and 1,0 ?B in Na1VP calculated using DFT. The bond length of V – O are 2,01 – 2,09 Å for Na4VP, 1,93 – 2,10 Å for Na3VP, 1,86 – 2,03 Å for Na2VP ,and 1,85 – 1,96 Å for Na1VP. The values above are in good agreement with pre-existing research for Na1VP and Na3VP. The redox mechanism doesn’t correspond well for Na2VP and Na4VP where the vanadium atoms form more than one oxidation state in the crystal structure due to stoichiometric constraints. Therefore, calculations only using PBE and PBEsol can give a good clue of a material’s properties however with some inaccuracies. This most likely is because PBE and PBEsol doesn’t properly account for localized electrons and strongly correlated electronic systems. Therefore, further studies should use exchange- correlation functional that better describes the electronic structure of materials, such as Hubbard correction or DFT+U method.