Transition metal dissolution from Li-ion battery cathodes

Yonas Tesfamhret
Abstract & Cover

Lithium-ion batteries (LIBs) have become reliable electrochemical energy storage systems due to their relative high energy and power density, in comparison to alternative battery chemistries. The energy density of current LIBs is limited by the average operating voltage and capacity of oxide-based cathode materials containing a variety of transition metals (TM). Furthermore, the low anodic stability of "conventional" carbonate-based electrolytes limits further extension of the LIBs voltage window. Here, ageing mechanisms of cathodes are investigated, with a main focus on TM dissolution and on strategies to tailor the cathode surface and the electrolyte composition to mitigate TM dissolution.
Atomic layer deposition (ALD) coatings of the cathode surface with electrically insulating Al2O3 and TiO2 coatings is employed and investigated as a method to stabilize the cathode/electrolyte interface and minimize TM dissolution. The thesis illustrates both the advantages and limitations of amorphous oxide coating materials during electrochemical cycling. The protective oxide layer restricts auto-catalytic salt degradation and the consequent propagation of acidic species in the electrolyte. However, a suboptimal coating contributes to a nonhomogeneous cathode surface ageing during electrochemical cycling. Furthermore, the widely accepted concept of charge disproportionation as the fundamental cause of TM dissolution is demonstrated to be a minor factor. Rather, a chemical dissolution mechanism based on acid-base/electrolyte-cathode interaction underlies substantial TM dissolution.
The thesis demonstrates LiPF6, and by implication HF, as the principal source of TM dissolution. In addition, the oxidative degradation of ethylene carbonate (EC) solvent contributes indirectly to generation of HF. Thus, an increase in electrolyte oxidative degradation products accelerates TM dissolution. Substituting EC and LiPF6 with a more anodically stable solvent (e.g., tetra-methylene sulfone) and a non-fluorinated salt (e.g., LiBOB or LiClO4) or addition of TM scavenging additives like lithium difluorophosphate (LiPO2F2) are here investigated as strategies to either i) mitigate TM dissolution, ii) supress TM migration and deposition on the anode surface, or iii) supress formation of acidic electrolyte degradation products and thereby TM dissolution. The thesis also highlights the necessity of taking precautions when attempting to replace the components, as reducing TM dissolution may come at the expense of electrochemical cycling performance.

Uppsala University
(Uppsala, Sweden)
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