Glucose-derived hard carbon electrode materials for sodium-ion batteries

Abstract

Naatrium-ioon akusid (NIB) on potentsiaalne tuleviku tehnoloogia taastuvenergia salvestamiseks. Na on laialdaselt levinud ning odav tooraine, samas on NIB-d suhteliselt kõrge energiatihedusega. Hard carbon tüüpi süsinikmaterjale peetakse seni parimaks negatiivse elektroodi materjaliks, kuid nende Na salvestamise mehhanism ja struktuuri ning elektrokeemiliste omaduste omavahelised seosed on endiselt vaieldav. Tahke elektrolüüdi piirpind (SEI), mis moodustub elektrolüüdi reduktiivsel lagunemisel negatiivse elektroodi pinnale ei ole nii stabliilne, kui liitium-ioon akudes. Käesolevas töös kasutatud süsinikmaterjal sünteesiti glükoosist hüdrotermilise karboniseerimise teel. Na salvestusmehhanismi uuriti röntgen hajumise meetodiga, SEI koostist massispektromeetriaga. Elektrokeemilise impedantsspektroskoopia meetodi abil võrreldi Li-, Na- ja K-elektrolüütide käitumist uuritud elektroodi pinnal. Leiti, et Na kemosorbeerub tugevalt materjali ning, et esimeses laadimise faasis toimub grafeeni kihtides C−C sideme pikenemine, mis viitab Na interkaleerumisele. Na- ja K-elektrolüüdid moodustavad väiksemal määral SEI-d kui Li, ning Na puhul on SEI kiht anorgaanilisem kui Li puhul. Kõige kõrgemad mahutavused antud elektroodimaterjaliga saavutati 1 M NaPF6 EC:PC (1:1) elektrolüüdiga – 350 mAh g−1 (millest 175 mAh g−1 platooalas) voolul 50 mA g−1. Viimaks varieeriti glütsiini-leeksünteesimeetodi parameetreid Na3V2(PO4)3 elektroodimaterjali sünteesiks, mida uuriti poolelementides ning lõpuks ka täiselemendis koos glükoosist sünteesitud materjaliga.

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.