Olivine LiFePO₄ is a major commercial cathode material for lithium ion batteries due to its high specific theoretical capacity, low cost, nontoxicity, and superb thermal/chemical stability. In addition to the above properties, olivine LiFePO₄ shows an exceptional high-rate capability despite its poor intrinsic electronic conductivity and sluggish two-phase reaction process. Last year, a non-equilibrium solid solution reaction process was experimentally demonstrated by Grey et al^[1]. Independent research was conducted by several groups and confirmed these results^[2],[3].

Recently, we have determined the crystal structure of intermediate Li_2/3FePO₄^[4]. However, until now there has been no investigation on the transport properties of the metastable intermediate phase formed at the phase boundary interface during the non-equilibrium process due to the following extreme technical difficulties.

First, the solid solution phase Li_xFePO₄ is metastable during electrochemical process and disappears within a few seconds after relaxation^[1,3]. We overcame this limitation by quenching Li_xFePO₄ (x = 0.6) at 350 °C to room temperature. This quenched phase remained stable for a couple of weeks, which enabled sufficient time to measure the intrinsic conductivity.

Second, conductive carbon may form during sintering at high temperature from possible carbon sources such as polyolefin worn from jars, organic solvent used in milling and carbon-containing precursors (e.g., oxalate FeC₂O₄·2H₂O).

Finally, impurities such as Li₃PO₄, Li₄P₂O₇ and Fe_xP resulting from off-stoichiometric mixing or the carbon reduction effect above 800 °C will largely increase the apparent conductivity of LiFePO₄ by several orders of magnitude^[5]. The scattering of the LiFePO₄ conductivity data resulting from the above extrinsic effects has troubled and confused scientists for a long time.

In our present study, pure carbon-free LiFePO₄ and FePO₄ were prepared using carbon-free precursors and controlling sintering parameters, and the intrinsic conductivity of quenched single phase Li_xFePO₄ (x = 0.6) was measured.

The following figure shows representative Nyquist plots of carbon-free LiFePO₄, carbon-free FePO₄, and quenched Li_0.6FePO₄ samples measured at 340 K. Of particular interest is that the electronic resistance of the quenched single-phase Li_0.6FePO₄ was approximately 2 orders of magnitude smaller than those of the two end-members (LiFePO₄ and FePO₄) at 340 K. The temperature-dependent conductivity was also measured. Across the entire temperature range of 30 to 350 °C, the single-phase Li_0.6FePO₄ shows approximately 2 orders of magnitude superior electronic conductivity. The activation energy of Li_0.6FePO₄ obtained from the slope of the temperature-dependent conductivity was almost identical to the two end members (LiFePO₄ and FePO₄), which means that the enhanced electronic conductivity originates from the increased charge carrier density. 

In summary, a metastable solid solution phase Li_xFePO₄ (x = 0.6) was isolated by the optimized quenching protocol and found to have approximately 2 orders of magnitude enhanced electronic conductivity over the two end members of LiFePO₄ and FePO₄. Our research reveals that the single-phase transformation mechanism not only relaxes the interface strain energy but also enhances the intrinsic charge transport, enabling the high-rate capability of olivine LiFePO_4.

References:

[1]      H. Liu, F. C. Strobridge, O. J. Borkiewicz, K. M. Wiaderek, K. W. Chapman, P. J. Chupas, C. P. Grey, Science. 2014, 344, 1252817–1252817.

[2]       Y. Orikasa, T. Maeda, Y. Koyama, H. Murayama, K. Fukuda, H. Tanida, H. Arai, E. Matsubara, Y. Uchimoto, Z. Ogumi, J. Am. Chem. Soc. 2013, 135, 5497–5500.

[3]       X. Zhang, M. van Hulzen, D. P. Singh, A. Brownrigg, J. P. Wright, N. H. van Dijk, M. Wagemaker, Nano Lett. 2014, 14, 2279–2285.

[4]       S. Nishimura, R. Natsui, A. Yamada, Angew. Chemie Int. Ed. 2015, 54, 8939–8942.

[5]       P. Herle, B. Ellis, N. Coombs, L. Nazar, Nat. Mater. 2004, 147–152.