Ronald Väli „Glucose-derived hard carbon electrode materials for sodium-ion batteries”

On 29 August at 14:15 Ronald Väli will defend his doctoral thesis  „Glucose-derived hard carbon electrode materials for sodium-ion batteries”.

Supervisors:
Prof. Enn Lust  (PhD), Institute of Chemistry, University of Tartu
Senior Research Fellow Alar Jänes (PhD), Institute of Chemistry, University of Tartu                      

Opponent:
Prof. Patrik Johansson, Chalmers University of Technology, Sweden

Summary:
Sodium-ion batteries (NIBs) have become potential candidates for large-scale stationary energy storage solutions due to sodium’s abundance and NIBs’ relatively high energy density. Although, hard carbon is one of the most promising negative electrode materials for commercial NIBs, its Na storage mechanism and structure−electrochemistry relationships are still debated. The protective solid electrolyte interphase (SEI) that forms on the negative electrode surface upon reductive decomposition of the electrolyte is not as stable as in commercial LIBs, which hinders the cycle life of NIBs.

In this thesis, glucose-derived hard carbon (GDHC) was synthesized via hydrothermal carbonization (HTC). Na storage mechanism into/onto hard carbon is studied using ex situ LA-ICP-MS and TOF-SIMS combined with galvanostatic charge-discharge (GCD) method. Differences in electrochemical behavior of Li, Na and K were evaluated using electrochemical impedance spectroscopy (EIS) and equivalent circuit fitting. Changes to the hard carbon structure during electrochemical cycling were evaluated using operando total X-ray scattering. Finally, the synthesis of Na₃V₂(PO₄)₃ positive electrode material via glycine-nitrate process (GNP) was described and analyzed and the performance of a GDHC||NVP full cell was demonstrated.

It was established that Na chemosorbs strongly into the hard carbon structure and that intercalation in the sloping region causes in-plane C−C bond elongation. Na- and K-based electrolytes consume less charge during SEI formation and Na-based SEI is more inorganic than Li-based SEI and contains less organic fragments. Highest half cell capacities were achieved with 1 M NaPF₆ EC:PC (1:1) electrolyte – 350 mAh g⁻¹ (of which 175 mAh g⁻¹ plateau) at 50 mA g⁻¹.