Separators play a critical role in lithium-ion batteries
(LIBs)
by facilitating lithium-ion (Li-ion) transport while enabling safe
battery operation. However, commercial separators made from polypropylene
(PP) or polyethylene (PE) impose a discrete processing step in current
LIB manufacturing as they cannot be manufactured with the same slot-die
coating process used to fabricate the electrodes. Moreover, commercial
separators cannot accommodate newer manufacturing processes used to
produce leading-edge microbatteries and flexible batteries with customized
form factors. As a path toward rethinking LIB fabrication, we have
developed a high-viscosity polymer composite separator slurry that
enables the fabrication of both freestanding and direct-on-electrode
films. A streamlined phase inversion process is used to impart porosity
in cast separator films upon drying. To understand the impacts of
material composition and rheology on phase inversion processing and
separator performance, we investigated four different separator formulations.
We used either diethylene glycol (DEG) or triethyl phosphate (TEP)
as a nonsolvent, and either silica (SiO2) or alumina (Al2O3) as an inorganic additive in a polyvinylidene
fluoride-co-hexafluoropropylene (PVDF-HFP) matrix.
Through a down-selection process, we developed a TEP-SiO2 separator formulation that matched or outperformed a commercial
Celgard 2325 (PP/PE/PP) separator and a Beyond Battery ceramic-coated
PE (CC/PE/CC) separator under rate and cycle life tests in LiFePO4|Li4Ti5O12 (LFP|LTO) and
LiNi0.5Mn0.3Co0.2O2|graphite
(NMC-532|graphite) coin cells at C/10–1C rates. Our TEP-SiO2 slurry had a viscosity of 298 Pa s at a 1 s–1 shear rate and shear-thinning behavior. When deposited directly
onto an LTO anode and cycled against an LFP cathode, the direct-on-electrode
TEP-SiO2 separator increased the specific capacity by 58%
and 304% at 2C rates relative to the PP/PE/PP and CC/PE/CC separators,
respectively. Additionally, the freestanding TEP-SiO2 separator
maintained dimensional stability when heated to 200 °C for 1
h and demonstrated a higher elastic modulus and hardness than the
PP/PE/PP and CC/PE/CC separators when measured with nanoindentation.
