Stable cycling of thin-film vanadium oxide electrodes between 4 and 0 V in lithium batteries
Introduction
The reversible insertion of lithium ions into host materials such as metal oxides and carbonaceous materials serves as the basic principle of today’s lithium-ion technology [1]. Much research has been devoted to understanding the structure–property relationship for a wide variety of metal oxides. It is well known that homogeneous topotactic reaction only takes place in certain a composition range, beyond which phase transitions occur. For example, crystalline vanadium oxide undergoes a series of phase transitions as the amount of lithium increases. Insertion of 3 moles of lithium into V2O5 results in the formation of ω-Li3V2O5, which has a sodium chloride type cubic structure and is found to be reversible [2]. Since this rock-salt structure does not have any vacancies to accommodate any more lithium, the ω-Li3V2O5 composition is considered to be the limit of the LiVO system.
Recently, LiMVO4 (M=Zn, Cd, Ni), and amorphous compounds such as RVO4 (R=In, Fe) and MV2O6+δ (M=Fe, Mn, Co) [3], [4], [5], molybdates [6], [7], and 3d-metal oxides [8], [9], [10], [11] have been investigated as higher-capacity alternatives to carbon anode materials in lithium-ion batteries. Initial charge capacities as high as 1300 mAh/g have been reported for FeVO4·2.7H2O, although the capacity quickly fades during cycling (61% of initial capacity retained after 10 cycles) [4]. The exact reaction mechanism is still a matter of debate. NMR and XAS analyses performed on deeply discharged Na0.25MoO3 indicate the formation of a lithium oxide-like compound as well as a LiMoO. Upon subsequent charging, the decomposition of “lithium oxide” seems to contribute to the reversible capacity [6], [7].
Very recently, Ali et al. demonstrated that thin-film Ni2V2O5 electrodes exhibit improved kinetics and cyclability compared to bulk materials [12]. We present our results on lithium insertion into amorphous vanadium oxide thin-film electrodes between 0 and 4 V. Our observations include a highly reversible cycling in this voltage range. Raman spectra of deeply discharged vanadium oxide thin films indicate dramatic structural changes between 1.5 and 1.8 V, after which the structural features remain largely intact. To our knowledge, this work represents the first investigation and analyses of extended cyclability data (nearly 1000 cycles) on deeply discharged metal oxide thin films.
Section snippets
Experimental
Vanadium oxide thin-film electrodes were prepared by plasma-enhanced chemical vapor deposition as described in a previous publication [13]. The vanadium source, VOCl3, was controlled at 10 °C and carried to the reaction chamber by argon at a pressure of 15 psi. The flow rates of VOCl3/Ar, H2, O2 were controlled at 500, 28 and 15 sccm, respectively. The temperature of the substrate, 430 stainless steel, was 25 °C. A 10 min deposition resulted in a film thickness of approximately 0.6 μm, as measured by
Results and discussion
Fig. 1 presents the capacity cycling data of a 0.6 μm thick PECVD vanadium oxide electrode in two different voltage ranges (4–1.5 and 4–0 V). The initial 15 cycles were performed between 4 and 1.5 V. Consistent with previously reported results [13], the electrode delivers a reversible capacity of 70 μAh/(μm cm2). Since the PECVD vanadium oxide has a density of 3 g/cm3, this volume capacity translates to 233 mAh/g. The cycling range was subsequently expanded to 0 V. An initial capacity of over 140 μAh/(μm
Acknowledgements
This work has been supported by the DDRD program at NREL, which is funded by the Department of Energy under Contract No. DE-AC-36-99GO10337.
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