Stabilizing strategies for the high‐voltage cathode material LiNi0.5‐xMn1.5+xO4 (LNMO)
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In order to reduce the amount of greenhouse gasses and stop global warming, the transition to a renewable energy-based society needs to be accelerated. The production of energy-dense and high-performing batteries is a key component in order to reach this goal. The Li-ion battery (LiB) is currently the best-performing battery technology in terms of energy density and cycle life. The large amounts of transition metals ™ like Ni and Co in the cathode active material, however, is leading to fluctuating prices and depletion of scarce and geographically limited resources. Making adjustments to reduce the use of scarce elements in LiBs is thus important to be able to produce more economically and environmentally friendly batteries. LiNi0.5−xMn1.5+xO4 (LNMO) is a promising high-voltage cathode material for LiBs, both due to the low price, low content of scarce resources, and its high energy density thanks to a high operating voltage. LNMO is, however, not yet implemented commercially on a large scale, due to issues concerning the reactive LNMO surface, high potential causing oxidative electrolyte degradation and TM cross talk, resulting in a limited cycle life. In this work, strategies to improve the stability of LNMO are investigated. The first part of the thesis, containing preliminary and unpublished work, deals with material synthesis of LNMO and spray drying as a possible coating technique. Phase pure LNMO was successfully synthesized by a modified Pechini method. Spray drying parameters were investigated, with the goal of obtaining surface-coated LNMO powders with optimized (pomegranate-like) morphology. A water-stability test revealed that Li loss and formation of an impurity phase occurs after water-exposure. Slurries containing LNMO and binders (polyvinyl butyral (PVB) and polyacrylic acid (PAA)) in isopropanol were tested. No observable change in particle morphology was detected. In the second part, Al2O3-coating of commercial LNMO powder by atomic layer deposition (ALD) was investigated as a stabilizing strategy. The chemically simple and abundant Al2O3 has previously been reported to improve the cycling stability of LNMO by prohibiting direct contact between LNMO and the electrolyte. Here, three coating thicknesses, estimated to be approximately 0.6, 1, and 1.7 nm were successfully deposited on the LNMO powder. LNMO with the thickest Al2O3 coating layer displayed poor cycling performance, and the cycling stability was not improved by the Al2O3-coating. Severe signs of pitting dissolution were furthermore discovered after long-term cycling at 50◦C. The surface degradation may be attributed to HF attack caused by side reactions between the electrolyte and the Al2O3-coating. The uniform and dense coating may lead to non-uniform conduction paths for Li, where the active sites experience high local current densities and are more susceptible to HF attack. In the third part, TiO2 was investigated as an alternative coating material to Al2O3. Three coating layers, estimated to be approximately 0.2, 0.3, and 0.6 nm, were successfully deposited on LNMO powder by ALD. Even though the TiO2-coating introduced some overpotential, improved cycling stability at room temperature was found for the TiO2-coated LNMO in a LNMO||graphite full-cell. This was attributed to the formation of a thinner cathode electrolyte interphase (CEI) and retardation of the TM dissolution. Based on these results, TiO2 was deemed a more promising coating material than Al2O3. In the fourth and final part, a room temperature ionic liquid (RTIL)-based electrolyte (1.2 M Lithium bis(fluorosulfonyl)imide (LiFSI) in N-Propyl-Nmethylpyrrolidinium bis(fluorosulfonyl)imide (PYR13FSI) (ILE)) was evaluated as an alternative to the carbonate-based commercial electrolyte (1 M LiPF6 in 1:1 ethylene carbonate (EC) / diethylcarbonate (DEC) (LP40)). RTILs are salts that are in a liquid state at room temperature and are much less flammable and volatile than the regular liquid organic electrolytes. Implementing RTIL electrolytes can thus simultaneously improve the battery safety and the cycling stability of LNMO. High anodic stability and improved cycling performance at 45◦C was found for ILE. The high ILE viscosity, however, leads to limitations in the rate performance at 20◦C.