Synthesis and Characterization of Highly-Conducting Nanocrystallized Li(Fe 1–x Mn x ) 0.88 V 0.08 PO 4 Cathode Materials (x = 0.25, 0.5, 0.75)

INTRODUCTION Since the pioneering publication by J.B. Goodenough et al. [1], phospho-olivines LiMPO 4 (M = Fe, Mn, Co, Ni) had been studied for their application as cathodes for Li-ion cells. From the whole family of isostructural compounds, only cathodes prepared from LiFePO 4 had been introduced into mass production. LiMnPO 4 has higher potential versus metallic lithium compared to LiFePO 4 , but synthesis of LiMnPO 4 compounds which can work in batteries with high loads is more difficult [2]. One of the successful synthetic routes consists in preparation of intentionally non-stoichiometric compositions [3]. Other possible way is to synthesize LiMn 1–x Fe x PO 4 phospho-olivines [4, 5]. In recent years, our group has proposed and investigated an alternative route to the conductivity enhancement, namely a thermal nanocrystallization of glassy analogs of the important crystalline cathode materials, such as: V 2 O 5 , LiFePO 4 and Li 3 V 2 (PO 4 ) 3 [6, 7]. The advantages of this approach include the absence of carbon additives, simplicity and straightforwardness of synthesis, which consists of two stages only: (i) glass preparation by melt-quenching and (ii) thermal treatment of the glass to induce its nanocrystallization. Our experience with that method has shown that one can, by the appropriate heat-treatment, achieve a gigantic (even by a factor 10 9 ) and irreversible conductivity enhancement. EXPERIMENTAL In this research, we aimed to replace some of iron ions in LiFe 0.88 V 0.08 PO 4 glass with manganese in order to obtain highly conductive nanomaterials. Appropriate amounts of precursors: Li 2 CO 3 , FeC 2 O 4 ·2H 2 O, Mn(CH 3 COO) 2 ·4H 2 O, (NH 4 )H 2 PO 4 and V 2 O 5 were mixed in a mortar, melted at 1300°C in reducing atmosphere and rapidly quenched. Their amorphousness was confirmed with X-ray diffractometry (XRD) and thermal events occurring in the samples were observed with differential thermal analysis (DTA). Electrical conductivity was measured upon heating and subsequent cooling ramps with impedance spectroscopy within wide frequency range 10 mHz – 10 MHz. RESULTS AND DISCUSSION DTA curves of as-received samples were typical for glassy materials. A glass transition and two crystallization peaks were observed. Preliminary electrical results (Fig. 1) showed that initial conductivity (1.2·10 –13 S/cm) can be increased of at least 10 orders of magnitude to ca 10 –3 S/cm. XRD patterns acquired upon heating to 580°C indicated three crystalline p