Layer-by-layer manufacture of improved energy storage devices
Keywords: deposition
pneumatic
aqueous
lithium
separator
manufacture
Li-ion
oxide
battery
structure
cracking
layered
structuring
cell
spray

A scalable layer-by-layer (LbL) spray processing technique has been investigated for the fabrication of electrodes and separators for lithium ion batteries (LIBs) with novel arrangements of constituent materials. Spray deposited electrodes based on LiFePO4 (LFP) had the same capacity (~148 mAh g-1), rate performance (50 mAh g-1 at 10 C) and electrochemical impedance spectroscopy response as those manufactured by conventional slurry casting. Slurry cast LFP electrodes from water-based slurries cracked at a thickness of ~10 μm and spontaneously delaminated from Al foil current collectors at a thickness of 70-80 μm (~10 mg cm-2) due to capillary pressure generated during drying. Spray deposited electrodes were crack-free up to at least 115 μm because of sequential drying of each, much thinner suspension layer, approximately equal to the average particle diameter in the suspension. A 115 μm (17.2 mg cm-2) thick spray deposited LFP-based electrode had a reduced capacity retention of 89% after 100 cycles when compared with thinner 6 mg cm2 spray deposited or slurry cast electrodes (both 92%), but provided a significant improvement in volumetric capacity at charge/discharge rates between 0.1 and 2C.
LFP-based electrodes with a through-thickness variation in local particle size and local porosity were produced. Whereas conventional LFP electrodes had a single voltage plateau during discharge, those containing two different particle size distributions had two distinct voltage plateaux. Over 200 0.5 C charge/discharge cycles the electrode made from a mixture of the two distributions lost 0.056% of its original 0.5 C capacity per cycle, while an electrode with higher porosity and smaller particle size next to the separator, and lower porosity and larger particle size next to the current collector, lost 0.046% capacity per cycle, an 18% improvement. An electrode with the "inverted" arrangement of identical materials lost 0.065% capacity per cycle, a 16% decrease. The electrode with increased porosity and lower particle size at the separator reduced overvoltage compared with other arrangements of the same materials at 10 C. The reduction in overvoltage was suggested to be due to greater Li ion concentration near the current collector and a lower utilisation of MTI particles (which had a higher degradation rate and greater polarisation). This work provides some of the strongest experimental evidence to support prior modelling of the performance of electrodes with non-uniform distributions of porosity and particle diameters.
A porous, spray deposited Al2O3-based separator was developed to enable the direct deposition of an electrode/separator/electrode full cell assembly in a single deposition operation. An optimised sprayed separator consisted of 50 nm Al2O3 particles, 1 wt. % polyacrylic acid (PAA) and 5 wt. % styrene butadiene rubber (SBR), deposited from an 80:20 suspension of water and isopropanol (IPA). Separators between 5 and 22 μm thick had consistent and similar porosity of ~58%, excellent wettability, thermal stability to at least 180 °C, adequate electrochemical stability and high ionic conductivity of ~1 mS cm-1 at room temperature, double that of a commercial PP separator. A fully spray deposited LFP/LTO full-cell with a 10 μm Al2O3 separator, the first of its kind, showed similar rate performance to a cell with a conventional PP separator, with a capacity of ~50 mAh g-1 at 30 C. On long term cycling of 400 cycles at 2 C, the Al2O3 separator cell had excellent capacity retention of 96.2% compared with 78.1% for the PP separator cell.
The flexible spray deposition approach has been shown to facilitate ultra-thick electrodes, layered electrodes that open a large design space for electrode structural design and improved performance, and relatively thin, porous inorganic separators with enhanced stability.