Effects of anti-site defects on reducing Li diffusion anisotropy in LiFePO₄ single crystal have been experimentally [1] and theoretically [2, 3] examined. The strongly anisotropic Li diffusion in olivine phosphates were firstly reported by first-principle calculations by Morgan et al. [4], and later confirmed by experiment [5]. However, the theoretical Li diffusivity (D=1x10^-8 cm²/s) in defect-free LiFePO₄ single crystals are several orders of magnitude larger than experimentally measured values in the range of 10^-11 – 10^-14 cm²/s. Subsequent calculations by Malik et al. [2] and Dathar et al. [3] confirmed that anti-site defects not only block the [010] diffusion channels but also decrease the energy barrier for inter-channel Li hopping, effectively reducing Li diffusion anisotropy. In particular, Amin et al. [1] reported the comparable Li diffusivities along the three axes in millimeter-sized LiFePO₄ single crystals containing 2.5-3% Li-Fe anti-site defects. Although the reduced Li diffusion anisotropy in LiFePO₄ single crystal is understood, the effects of anti-site defects on phase transformation kinetics in olivine LiFePO₄ particles have not been clarified. Here we show that both the phase transformation kinetics and the surface reaction kinetics of LiFePO₄ particles will be significantly affected by the anti-site defects.

Recently, we combine operando hard X-ray spectroscopic imaging and phase-field modeling to show two-dimensional (2D) Li diffusion behaviors in micro-sized LiFePO₄ rod containing ~3% Li-Fe anti-site defects. We obtain direct evidence that Li ions can be intercalated through the (100)/(001) surfaces, contradicting a common belief that only the (010) surface of LiFePO₄ is electrochemically active. This study not only presents the first experimental confirmation of the previously hypothesized surface-reaction-limited (SRL) [6] phase boundary migration in LiFePO₄, but also reveals a new hybrid mode of phase transformation, where the growth of new phase is controlled by surface reaction or Li diffusion in different crystallographic directions. Based on these findings, we propose the existence of three distinct kinetic regimes (SRL, hybrid, and bulk-diffusion-limited (BDL)) of phase transformations of LiFePO₄ and potentially many other intercalation compounds undergoing first-order phase transformations (as shown in Figure 1a). In particular, our phase-field simulations show that 2D Li diffusion can significantly enhance Li insertion kinetics in SRL regime.

In a LiFePO₄ nanoparticle of 200 nm x 50 nm x L nm ([100] x [010] x [001]), our simulation results (Figure 1b) show that the traveling constant velocity of (100) straight phase boundary driven by 2D Li diffusion (D=1x10^-11 cm²/s along [100] and [010]) is one magnitude larger than the velocity of the phase boundary driven by 1D Li diffusion with infinite diffusivity along [010]. The latter one is calculated by SCB theory [6]. Anti-site defects are commonly believed to deteriorate high rate performances of LiFePO₄, due to its blocking to the 1D Li diffusion along [010]. Our counterintuitive finding, however, show that the enhanced Li diffusion along [100] by Li inter-channel hopping around the defects [2, 3] significantly enhance surface reaction kinetics by activating large fraction of electrochemically surface for Li insertion. The maximum phase boundary velocity driven by 2D Li diffusion is linear to particle size along [100] and inversely linear to thickness along [010]. Therefore, we propose that LiFePO₄ nanosheets with large (010) surface and small thickness along [010] is beneficial for high rate capability and high energy density. This is consistent to experimental observations.

Our studies show the important effects of anti-site defects on phase transformation kinetics and surface reaction kinetics in LiFePO₄. This will provide new insights on potential avenues to improve performances of olivine electrodes via engineering anti-site defect formation and distribution.

References:

1. Amin, R.; Maier, J.; Balaya, P.; Chen, D. P.; Lin, C. T. Ionic and electronic transport in single crystalline LiFePO₄ grown by optical floating zone technique. Solid State Ionics 2008, 179, 1683-1687.
2. Malik, R.; Burch, D.; Bazant, M.; Ceder, G. Particle size dependence of the ionic diffusivity. Nano Lett. 2010, 10, 4123-4127.
3. Dathar, G. K. P.; Sheppard, D.; Stevenson, K. J.; Henkelman, G. Calculations of Li-ion diffusion in olivine phosphates. Chem. Mater. 2011, 23, 4032–4037.
4. Morgan, D.; Van der Ven, A.; Ceder, G. Li Conductivity in Li_xMPO₄ (M=Mn, Fe, Co, Ni) olivine materials. Electrochem. Solid State Lett. 2004, 7, A30.
5. Nishimura, S.; Kobayashi, G.; Ohoyama, K.; Kanno, R.; Yashima, M.; Yamada, A. Experimental visualization of lithium diffusion in Li_xFePO₄. Nature Mater. 2008, 7, 707.
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