Long-term
cycling studies of high capacity Li-metal|lithium iron phosphate (LFP,
3.5 mAh/cm2) cells were carried out using two highly concentrated
ionic liquid electrolytes (ILEs). Cells incorporating N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide
(C3mpyrFSI) or triethylmethylphosphonium bis(fluorosulfonyl)imide
(P1222FSI), with 50 mol % lithium bis(fluorosulfonyl)imide
(LiFSI) electrolytes were shown to operate for over 180 cycles at
50 °C at a rate of C/2 (1.75 mA/cm2). The choice of
separator was identified as a critical factor to enable high areal
capacity cycling, with the occurrence of cell failure through a short-circuiting
mechanism being particularly sensitive to separator characteristics.
Several commercial separators were characterized and tested, and their
performance was related to membrane properties such as the MacMullin
number, pore size, and contact angle. Celgard 3000 series separators
were found to support long-term cycling due to their combination of
desirable nanoporosity and wettability. The most compatible cell components
were assembled into a pouch cell to further demonstrate the feasibility
of ILE incorporation into high-capacity lithium metal batteries for
commercial purposes.
High
performance Li|NMC and Li|LFP cells using ionic liquid-based
electrolytes have previously been demonstrated, whereby the choice
of commercial polyolefin separator was found to play a determinative
role in the lithium metal anode’s cycling performance and stability.
Here, the relationship between the separator properties and the lithium
metal cycling behavior has been explored by considering the role and
importance of electrolyte chemistry and its resultant interactions
with the separator and electrode surface. In this study, an ionic
liquid electrolyte (ILE) based on the bis(fluorosulfonyl)imide (FSI)
anion was chosen, namely N-methyl-N-propylpyrrolidinium FSI (C3mpyrFSI) with 3.2 mol·kg–1 LiFSI. An organic:IL hybrid electrolyte consisting
of 20:80 DME:IL (with the same respective LiFSI molalities) was also
prepared using the same IL. Five separators were investigated, namely
Solupor 7P03A, Solupor 5P03A, Celgard 3501, Celgard 3401, and Celgard
2500, and the combinations of electrolyte-separator were characterized
ex situ, both before and and after cycling, in a Li|Li symmetric coin
cell. The interaction between the separator and electrolyte and the
subsequent impact on the electrolyte transport properties have been
characterized using NMR diffusion and electrochemical impedance spectroscopy
measurements (MacMullin number). The evolution of the lithium metal
morphology was studied using SEM, revealing the degree to which the
deposited lithium grows progressively into the separator during cycling.
Identifying the key properties of the electrolyte-separator system
(e.g., ion transport) and understanding how they can be modified through
choice of separator and electrolyte chemistry to improve the lithium
metal anode durability is a critical aspect for the future development
of durable, high capacity lithium metal batteries.
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