Short communicationPhase formation and electrochemical properties of cryochemically processed Li1+xV3O8 materials
Introduction
Li1+yV3O8 (y ≈ 0.2) is a lithium vanadium bronze considered as one of promising cathode materials for rechargeable lithium batteries due to its low cost, high electronic conductivity and potentially high specific electrochemical capacity [1], [2]. It is known that upon Li uptake (x) Li1+xV3O8 undergoes structural modifications accompanied by a second-order phase transition from the low-lithiated form (ca. Li2.9V3O8) into the high-lithiated one (ca. Li4V3O8) [3], [4]. Accordingly, the process of Li electrochemical intercalation into Li1+yV3O8 can be divided into three steps: single phase Li intercalation into low-lithiated phase, coexistence of the two phases (1.9 < x < 3.0) and single phase Li intercalation into Li4V3O8 [5]. Due to the structural difference between the two forms of Li1+xV3O8 the lithium insertion process is not totally reversible; a little alteration of the initial structure after repetitive electrochemical cycling was observed [6]. It was also shown that the transformation of Li1+xV3O8 from one structural type to the other one can occur at different x values [5], [6]. Another important feature of the Li intercalation process is a significant decrease in Li+ diffusion rate at high Li content [7]. All these factors point to the crucial influence of kinetic factors on the lithium diffusion and, hence, to the completely different electrochemical performance of Li1+xV3O8 materials obtained by different methods.
The traditional methods of Li1+yV3O8 synthesis are based on the slow cooling of melt, obtained from V2O5 and Li2CO3 at 680–900 °C [8], [9], [10]. This preparation route leads to the reproducible formation of perfectly crystalline faceted Li1+yV3O8 particles; amorphous quenching products demonstrated a much poorer electrochemical capacity. However, due to the large grain size and above-mentioned kinetic complications of Li+ mobility at high x values these materials usually demonstrate a moderate electrochemical capacity (<180 mAh g−1) often accompanied by a high fade rate.
The most of modern Li1+xV3O8 studies deal with the chemical methods of powder synthesis. The decrease in particle size diminishes Li+ diffusion pathways, and thus ensuring a better electrochemical performance [7]. It was shown that the soft dehydration of the gel obtained by dissolution of V2O5 in the aqueous LiOH results in the formation of poorly crystalline product with individual structure, identified as polymorph of LiV3O8 [11] or LiV3O8·(H2O)z [12], that can accommodate over 4 Li per formula unit [11], [13]. Usually the highest initial specific capacity values (>300 mAh g−1) are demonstrated by the Li1+xV3O8 materials with ordinary monoclinic structure of the γ-lithium vanadate [3] obtained by the thermal processing of various precursors at 300–400 °C, though an important capacity fade rate upon cycling is often observed [14], [15]. Thermal processing performed below 300 °C is usually insufficient for the complete removal of intermediate decomposition products while the increase in thermal processing temperature over 400 °C causes the progressive degradation of electrochemical performance [16], [17], [18], that can be attributed to the intensive grain growth and related kinetic complications of Li+ diffusion.
Different synthesis methods lead to substantially different Li1+xV3O8 crystallization rates, so that the crystallinity degree of powders, obtained at 300–400 °C, is rather different; particle size, morphology and packing are also strongly influenced by synthesis routes [19], [20], [21], [22]. However, the electrochemical performance of almost amorphous and well-crystallized powders obtained in this temperature range is often very similar and close to 250–270 mAh g−1 at a moderate discharge rate [14], [17], [18], [21].
Meanwhile, the considerable discrepancy of results obtained by various authors clearly demonstrates that numerous factors controlling the electrochemical behavior of Li1+xV3O8 fine powders still have to be revealed. Taking account of restrained information concerning the influence of chemical prehistory on the properties of Li1+xV3O8-based cathode materials and the versatility of cryochemical processing methods [23], this study is devoted to the freeze-drying synthesis of Li1+xV3O8 powders from different salt precursors and to the investigation of the phase formation, microstructure and electrochemical properties of the final materials.
Section snippets
Experimental
The most of LiV3O8 materials in the present study were prepared from the LiNO3·H2O (Aldrich, 98%) + NH4VO3 (Aldrich, 99+%) precursor (further denoted as LV1). Several experiments were performed using CH3COOLi·2H2O (Aldrich, 99.999%) + NH4VO3 (LV2) and HCOOLi·H2O (Aldrich, 98+%) + NH4VO3 (LV3) precursors. Aqueous solutions containing Li and V salts in molar ratio 1.2:3 were sprayed into liquid nitrogen by pneumatic nozzle and freeze dried at 5 × 10−2 mbar for 48 h at T ≤ 42 °C. The pH of the starting
Results and discussion
The phase composition of freeze dried salt precursors is one of the key factors controlling the formation mechanism and morphology of the freeze drying synthesis products [23]. Indeed, XRD patterns of the LiV3O8 precursors obtained from starting solutions with different anionic compositions were found considerably different. All the freeze-dried precursors contained various polyvanadates while LV2 and LV3 precursors contained also hydrated lithium acetate and formate, respectively. In LV1 the
Conclusions
The freeze drying of aqueous solutions containing NH4VO3 and various Li compounds results in the formation of salt precursors containing polyvanadate anions which considerably differ from those existing in aqueous medium. The decrease in the pH of starting solution promotes the formation of NH4V3O8, which is crystallographically similar to LiV3O8. Thus, its formation is facilitated during the thermolysis, and the thermal decomposition of freeze dried precursors results in the formation of LiV3O8
Acknowledgements
The authors are indebted to Dr. Serge V. Pushko for the fruitful discussion and to J. Garden (INPG, Grenoble) for taking SEM micrographs.
References (26)
J. Inorg. Nucl. Chem.
(1965)- et al.
Solid State Ionics
(1993) - et al.
J. Solid State Chem.
(2005) - et al.
J. Power Sources
(1999) - et al.
Solid State Ionics
(1999) - et al.
Solid State Ionics
(1999) - et al.
Solid State Ionics
(1984) - et al.
J. Power Sources
(1999) - et al.
J. Power Sources
(1995) - et al.
Mater. Lett.
(2003)
J. Power Sources
Electrochim. Acta
Electrochim. Acta
Cited by (0)
- 1
Present address: Thin Film Materials Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, 136-791 Seoul, Korea. Tel.: +82 2 958 5554; fax: +82 2 958 5554.