Enhanced Conductivity in the Metastable Intermediate in Li
x
FePO4 Electrode
Authors: Lu Jiechen , Oyama Gosuke , Nishimura Shinichi , Yamada Atsuo 
Published: February 2020

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 Li2/3FePO4
[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
x
FePO4 is metastable during electrochemical process and disappears within a few seconds after relaxation[1,3]. We overcame this limitation by quenching Li
x
FePO4 (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
x
P 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
x
FePO4 (x = 0.6) was measured.

The following figure shows representative Nyquist plots of carbon-free LiFePO4, carbon-free FePO4, and quenched Li0.6FePO4 samples measured at 340 K. Of particular interest is that the electronic resistance of the quenched single-phase Li0.6FePO4 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 Li0.6FePO4 shows approximately 2 orders of magnitude superior electronic conductivity. The activation energy of Li0.6FePO4 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
x
FePO4 (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 LiFePO4.

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.

Figure 1