Electrochemical Kinetics of Carbon-Nested Ultrafast Nano-LiFePO4 for Hybrid Ees
Authors: Kisu Kazuaki , Iwama Etsuro , Nakashima Shota , Sakai Yuki , Orikasa Yuki , Leone Philippe , Dupré Nicolas , Naoi Wako , Brousse Thierry , Rozier Patrick , Simon Patrice , Naoi Katsuhiko 
Published: February 2020

1. Introduction

Olivine phosphates have long been investigated as cathode material in Li-ion batteries. In particular, lithium iron phosphate (LiFePO4) has attracted the attention of many researchers because of its high theoretical capacity of 170 mAh g-1, low cost and high electrochemical/thermal stabilities of the phosphate (PO4
3-) anion.[1] However, the large volume difference during two-phase reaction between Li-rich Li1-aFePO4 (LFP) and Li-poor LibFePO4 (FP) phases leads to lattice mismatch at the LFP/FP boundary which induces a low Li-ion diffusion coefficient of 10-14 cm2 s-1. [2] Such 1D Li+ diffusivity in the olivine-LFP is much slower (by 5-8 orders of magnitude) as compared with that of LCO (10-9 cm2 s-1) and LMO (10-5 cm2 s-1) cathodes. As a result, this slow lithium ion diffusion and the relatively poor electronic conductivity (10-10 to 10-7 Ω-1 cm-1) of the olivine-LFP in the absence of a doping cation, limit the power capability of the LFP material. To overcome these limitations, we have proposed new concept of three-phase structure containing a core of crystalline LFP, a shell of amorphous LFP, and graphitic carbon derived from Ketjen Black (KB) synthesized using our original in-situ ultracentrifugation process (UC process) [3]. The peculiar core-shell structure of LFP nanoparticle within the graphitic carbon prepared by UC-process improves the electronic conductivity in the whole LFP/graphitic carbon composites, while the amorphous LFP phase at the particle surface (shell) can achieve excellent rate performance owing to its high Li-ion diffusion coefficient.

2. Experimental

A precursor solution was prepared by mixing KB and H3PO4 aq. in ultra-pure water. The precursor solution was treated by UC process after an addition of iron acetate and lithium acetate. After drying at 80°C in vacuum for 12 h, the precursor composite powder was obtained. The powder was lastly annealed at 700 ◦C under N2 flow for 5 min and the final product (LFP/ graphitic carbon composite powder) was obtained. The LFP/graphitic carbon composite electrode was electrochemically characterized using a 2032 coin half-cell with Li metal in 1M LiPF6/EC+DEC (vol. 1:1).

3. Results and Discussion

The electron microscopy observations show that LFP/graphitic carbon composite has a highly crystalline phase of LFP core of ca.12-15 nm diameter with a distance of d=0.32 nm corresponding to (110) plane of the olivine LFP. This crystalline core, that we called “core 1”, is entirely covered with amorphous layers (referred as “core 2”) in light grey in the TEM image. The combination of TEM observation together with XRD, XPS and Mössbauer analysis, supports the hypothesis that amorphous LFP contains Fe3+ defect. The most outer layer (“shell”) is composed of random carbon (KB) fragments/sheets stacked onto each other. The thickness of the shell is about 5 nm. An interlayer distance of d=0.35 nm was measured which is a little larger than that of graphene. This LFP/graphitic carbon composite enabled a 100C rate (36 seconds) discharge with 60 mAh g-1 per composite corresponding to 70% of the capacity obtained at the slowest discharge rate (1C). An amorphous LFP containing Fe3+ defects enhances lithium ion diffusion coefficient probably by switching from one-dimensional to three-dimensional diffusion. Then, the electron utilized in Fe redox reaction can be fast transferred by the covering graphitic carbon coating the particles. Such an ultrafast charge–discharge performance opens the possibility of using LiFePO4 as a cathode material for ultrafast hybrid capacitor with a stable cycle performance over 2,000 cycles at a 10C rate, maintaining 90% of the initial capacity.

References

1) A. K. Padhi, et al., J. Electrochem. Soc., 1997, 144, 1188-1194.

2) G. Kobayashi, et al., Adv. Funct. Mater., 2009, 19, 395-403.

3) K. Naoi, et al., J. Electrochem. Soc., 2015, 162 (6), A1-A7.