Abstract
The LiFePO4 (LFP) cathode has a theoretical specific capacity of 170 mAh/g. This material is stable, safe, and environmentally benign. Li-ion batteries with LFP cathodes have a long cycle life with excellent charging/discharging performance. In our contribution, we show that the surface modification of the LFP cathode using ultrathin alumina films grown by atomic layer deposition (ALD) can improve Li-ions charge transfer and rate performance of the Li-ion in half-cell configuration with the LFP cathode.
We used two types of LFP cathodes in our study with Li-metal as an anode in a half cell configuration. The first LFP electrode had a thickness of 25 μm and consisted of particles of 0.5 - 1 μm in size [1], while the second one (commercial NANOMYTE BE-60E (NEI Corp.)) had a thickness of 70 μm and the average LFP particle size was 2 μm.
Ultrathin Al2O3 films were grown on LFP cathodes by ALD at 100 °C using trimethylaluminum (TMA) and water vapors. Due to the self-saturation surface-limited nature of the ALD reaction, even substrates of highly complex morphology can be uniformly coated, where the limitation is the reach of the precursor vapor molecules. The thickness of the ALD layers depends linearly on the number of ALD cycles and ranges from 0.5 to 2 nm for 5 to 20 cycles.
Galvanostatic charging/discharging experiments were performed to evaluate the rate capability of the electrodes. Pristine and Al2O3 coated LFP electrodes were laser cut to 18 mm diameter circular electrodes, inserted in the electrochemical test cells PAT-Cell (EL-CELL), and tested using PAT-Tester-x-8 analyzer. LFP with Al2O3 protecting layers was compared to the pristine LFP electrode.
For the LFP cathode with the thickness of 25 μm the pristine sample saturates at the specific capacity of 32 mAh/g at 1.8 c-rate, while for the electrode treated with 5 and 20 cycles of Al2O3 the specific capacity dropped to 80 and 60 mAh/g, respectively. The electrodes protected by Al2O3 film always exhibited better rate capability than the pristine sample.
A more detailed analysis of the performance was accomplished for the NANOMYTE electrodes. A specific discharge capacity of 160 mAh/g at a c-rate of 0.2 was achieved for both pristine and Al2O3-protected electrodes. Two supercycles with the c-rate 0.2, 0.5, and 1 were applied to the samples. The specific capacity of the pristine LFP electrode dropped from 160 mAh/g for the c-rate 0.2 to about 90 mAh/g at the c-rate 1, while the electrode which was protected by 5 cycles of Al2O3 exhibited a capacity of 120 mAh/g.
Investigation of the rate capability fading using electrochemical impedance spectroscopy revealed a gradual increase of the Nyquist plot semicircle diameter as a function of cycles. According to the equivalent circuit model, the semicircle diameter corresponds to the charge transfer at the solid/electrolyte interface. The charge transfer resistance at the solid/electrolyte interface increases with charge/discharge cycling. The charge transfer resistance of the pristine electrode increased more steeply than that of the electrode with 5 cycles of Al2O3. Improvement of the charge transfer resistance of the LFP electrode covered by thin Al2O3 film can be explained by the formation of the Li3.4Al2O3 phase upon lithiation. The diffusivity of the Li ions in the Li3.4Al2O3 phase is several orders of magnitude higher than that of in the Al2O3 film [2]. The results obtained by electrochemical characterization are compared to the X-ray photoelectron spectroscopy performed on samples before and after galvanostatic charging/discharging measurements.
[1] A. Fedorkova et al., J. Power Sources 195 (2010) 3907.
[2] S. C. Jung et al., J. Phys. Chem. Lett. 4 (2013) 2681.