Article pubs.acs.org/cm
Unusual Conversion-type Lithiation in LiVO3 Electrode for LithiumIon Batteries Jeong Beom Lee,† Janghyuk Moon,‡ Oh B. Chae,† Jae Gil Lee,† Ji Heon Ryu,§ Maenghyo Cho,‡ Kyeongjae Cho,∥ and Seung M. Oh*,† †
Department of Chemical and Biological Engineering and Institute of Chemical Processes and ‡WCU Multiscale Mechanical Design Division, Department of Mechanical and Aerospace Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea § Graduate School of Knowledge-based Technology and Energy, Korea Polytechnic University, 237 Sangidaehak-ro, Siheung-si, Gyeonggi 15073, Republic of Korea ∥ Department of Materials Science and Engineering and Department of Physics, The University of Texas at Dallas, Richardson, Texas 75080, United States S Supporting Information *
ABSTRACT: This work finds that LiVO3 is lithiated by a conversion reaction at 25 °C, which is unusual for the family of vanadium oxides. The spectroscopic studies and first-principle calculations performed on the lithiation mechanism of LiVO3 consistently propose that a two-phase insertion-type lithiation proceeds in the early stage of lithiation; LiVO3 transforms into a rock-salt structured Li2VO3. The continuing single-phase Li+ insertion into the tetrahedral sites in the rock-salt Li2VO3 produces a more Li-rich phase (Li2.5VO3), which is highly distorted because of the unfavorable Li+ insertion into the tetrahedral sites such as to be vulnerable to lattice breakdown. Hence, a two-phase (nucleation/growth type) conversion reaction is followed along with a structural disintegration; the Li2.5VO3 phase decomposes into metallic vanadium and Li2O. To determine the factors facilitating the conversion reaction of LiVO3, galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) are performed on LiVO3, the results of which are then compared to those observed with V2O5, which is not lithiated by the conversion reaction at 25 °C. The results show that the quasi-equilibrium potential for the conversion reaction is more positive for LiVO3 (thermodynamically more feasible). Also, the conversion reaction is kinetically more facilitated for LiVO3 due to faster solid-state diffusion of mobile ionic species during the two-phase growth stage of metallic vanadium and lithium oxide (Li2O) in the conversion process.
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INTRODUCTION
defined interstitial sites of crystalline lattices or in structural defects being populated in amorphous metal oxides.3−5 The metal ions in metal oxides play as the electron hosts (redox centers). Because Li+ ions are stored/released without structural disintegration, the volume change upon cycling is not serious to give rise to a stable cycle performance. This merit is, however, largely offset due to limited Li+ storage capacity resulting from the well-defined but limited Li+ storage sites in crystalline lattices. The metal oxides lithiated by conversion reaction show a somewhat opposite behavior to the insertiontype metal oxides. Here, the lithiation is triggered by metal− oxygen bond cleavage. Then, the injected electrons convert the metal ions (redox centers) to their elemental states, while the
Lithium-ion batteries (LIBs) have been used in wide applications because of their high energy density compared to other battery systems.1 Stable intercalation chemistry of graphite at 0.2−0.3 V (vs Li/Li+) has made it possible for LIBs to play a major role in electrical energy storage.2 However, for this battery system to meet the requirements of highly demanding devices such as electric vehicles (EVs) and electric energy storage system (ESS), the limitation of graphite, the specific capacity of 372 mA h g−1, should be overcome. As an alternative to graphite, metal oxides have been exploited, in which the lithiation proceeds by a coinjection of Li+ ions and electrons. For lithiation, metal oxides should thus carry the hosts for both Li+ ions (Li+ storage sites) and electrons (redox centers). The Li+ storage mechanism in metal oxides can be divided into two types: insertion and conversion. In the former-type lithiation, Li+ ions are inserted into the well© 2016 American Chemical Society
Received: March 14, 2016 Revised: July 4, 2016 Published: July 5, 2016 5314
DOI: 10.1021/acs.chemmater.6b01053 Chem. Mater. 2016, 28, 5314−5320
Article
Chemistry of Materials coinjected Li+ ions are stored as lithium oxide (Li2O).6−8 Note that the as-generated metal and lithium oxide are nanosized, such that the reverse reaction is quite feasible because the contact area between the two components is large and solidstate diffusion length is short.8−10 In general, the specific capacity of conversion-type oxides, which is determined by the oxidation state of metal ions (for example, four Li+ ions/ electrons per formula unit of MO2), is generally larger than that of the insertion-type oxides. However, volume change is severe and cycle performance is poor. The previous reports read that the experimentally observed potentials for conversion reaction, which are the transient values combining the thermodynamic value (equilibrium potential for conversion reaction) and polarization (kinetic value), show a rough proportionality to atomic number for the 3d transition metal oxides (Figure S1). This feature can be explained on the basis of thermodynamic consideration. The thermodynamic reduction potential of early transition metal ions is lower (more negative) than that for the later transition metal ions.11 Thus, the conversion-type lithiation is not thermodynamically favored for the early transition metal oxides. Meanwhile, the conversion reaction is kinetically hindered due to a large activation barrier for the diffusion/ rearrangement of mobile species such as Li+, Mn+, and O2− ions.12 In short, in the case of early transition metal oxides (for example, titanium oxides and vanadium oxides), the conversion reaction is not thermodynamically favored and kinetically hindered. As is seen in Figure S1, the potential for conversion reaction for titanium oxides and vanadium oxides does not appear at >0.0 V (vs Li/Li+) but seems to locate at
