LiNi0.5Mn1.5O4 cathodes for lithium-ion batteries : Exploring strategies for a stable electrode-electrolyte interphase

Sammanfattning: Climate change, a pressing global issue, can be partially addressed by using electric vehicles to reduce CO2 emissions. In this context, high-energy and high-power density batteries are vital. The LiNi0.5Mn1.5O4 (LNMO)-based cell is in this regard appealing as it fulfils several requirements, but is unfortunately constrained by capacity fading, especially at elevated temperatures. LNMO operates at ~ 4.7 V (vs. Li+/Li) at which conventional Li-ion battery (LIB) electrolytes are not thermodynamically stable.This thesis investigates the degradation mechanisms in LNMO cells and various practical strategies to tackle these problems. In the first part, a technique named synthetic charge-discharge profile voltammetry (SCPV) is developed to better understand the oxidative stability of some of the common electrolytes. The second part focuses on the use of binders that could potentially enable the formation of an artificial cathode-electrolyte interphase in LNMO cells. Polyacrylonitrile (PAN), which is often considered to be oxidatively stable, is however shown to degrade under the operating voltages of LNMO. A second polymer, polyacrylic acid (PAA), was studied for higher electrode mass loadings, but a high internal resistance resulted in poor initial discharge capacity as compared to the carboxymethyl cellulose (CMC) benchmark.In order to effectively mitigate capacity fading, three different electrolytes were explored in LNMO cells in the third section. First, an ionic liquid-based electrolyte, 1.2 M lithium bis(fluorosulfonyl)imide (LiFSI) in N-Propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR13FSI), was used. X-ray photoelectron spectroscopy (XPS) analysis revealed that this electrolyte stabilized the electrode by forming robust and predominantly inorganic surface layers which stabilized the electrode. Second, the study of an electrolyte containing sulfolane showed that, despite initial cycles displaying a higher degradation, the passivation layers created on the electrodes enable stable cycling. In a third study, tris(trimethylsilyl)phosphite (TMSPi) and lithium difluoro(oxalato)borate (LiDFOB) were investigated as electrolyte additives in a conventional electrolyte, and 1 wt.% and 2 wt.% of the additives, respectively, showed improved electrochemical performance in LNMO-graphite full cells, highlighting the role of these additives in enabling interphase layers at both the positive and negative electrodes. Collectively, these studies offer insights on how crucial the interfacial chemistry is for stable operation of LNMO cells, and pinpoint strategies to tailor this further.