NMR Study on Li þ Ionic Motion in Li x V 2 O 5 (0:4 5 x 5 1:4) Daisuke NISHIOKA , Koichi NAKAMURA, Yoshitaka MICHIHIRO, Takashi OHNO, Murugesan VIJAYAKUMAR 1 , Subramanian SELVASEKARAPANDIAN 1 , and Hiroyuki DEGUCHI 2 Department of Quantum Materials Science, Institute of Technology, The University of Tokushima, Tokushima 770-8506 1 Solid State and Radiation Physics Laboratory, Department of Physics, Bharathiar University, Coimbatore-641 046, Tamilnadu, India 2 Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550 (Received August 9, 2007; accepted December 7, 2007; published February 12, 2008) The temperature dependences of the 7 Li NMR line width and spin–lattice relaxation rate 1=T 1 have been measured in lithium vanadium bronze Li x V 2 O 5 with 0:4 5 x 5 1:4 over the temperature range of 77 – 550 K. The narrowing of has been observed for each sample, which is interpreted in terms of the motional narrowing due to the Li þ ionic diffusion. In Li 0:8 V 2 O 5 with a single -phase, an activation energy E m for the hopping of Li þ ions is estimated to be 0:16 0:05 eV, whereas in Li 0:4 V 2 O 5 the value of 0:10 0:05 eV is deduced for the -phase. The ionic conductivity evaluated from the diffusion coefficient D in Li 0:8 V 2 O 5 at 400 K is 6:3 10 7 1 cm 1 , which is consistent with the reported value of dc ’ 10 7 1 cm 1 . Below 300 K, 1=T 1 is considered to be due to magnetic interaction of 7 Li nuclear spins with V 4þ electronic spins. Above 350 K, 1=T 1 is dominated by Li þ ionic diffusion, from which E m is estimated to be 0:10 0:05 eV. KEYWORDS: lithium vanadium bronze, Li þ ionic diffusion, ionic conductivity, NMR, motional narrowing, spin– lattice relaxation DOI: 10.1143/JPSJ.77.024602 1. Introduction Recent years there has been a growing interest in lithium ion rechargeable battries, because the portable electric devices such as mobile telephones, laptop computers need the batteries with high energy density. The capacity of lithium ion batteries is closely related to the cathode electrode in which lithium ions can be inserted reversibly over a wide solid-solution range. In commercial recharge- able lithium batteries, LiCoO 2 has been widely used as the cathode electrode. However, LiCoO 2 is so expensive that exchangeable materials with higher electric performance have been investigated. One of the promising materials is the lithium vanadium bronze Li x V 2 O 5 in which a host V 2 O 5 can intercalate as much as about two Li þ ions. 1,2) Li x V 2 O 5 shows various crystalline phases such as -, -, "-, -, and -phases depending on the Li þ ions concentra- tion and synthesizing procedures. The host V 2 O 5 has an orthorhombic structure with the V 2 O 5 layers consisting of edge- and corner-sharing VO 5 square pyramids parallel to the ab plane. The phase ðx < 0:1Þ, the þ " coexisting phase ð0:1 < x < 0:35Þ, the " phase ð0:35 < x < 0:7Þ, the " þ coexisting phase ð0:7 < x < 0:8Þ and the -phase ð0:8 < x < 1Þ are obtained successively by the electro- chemical lithium insertion into V 2 O 5 at room temper- ature. 2,3) The boundary Li þ ion composition of each phase differs slightly between refs. 2 and 3. Up to x ¼ 1, lithium can be intercalated with a good reversibility, and the structure of the V 2 O 5 layer is approximately maintained, though a slight puckering of the V 2 O 5 layer occurs with increasing lithium content. For x > 1, the -phase is progressively replaced by the -phase, which is formed irreversibly and has the structure with more pronounced puckering of V 2 O 5 layer. On the other hand, when Li x V 2 O 5 is synthesized at high temperatures above 350 C, the -phase is formed approximately in the region, 0:2 < x < 0:6. -LiV 2 O 5 has a monoclinic structure and is different from the -, "-, -, and -phases obtained through the electrochemical procedure at room temperature. 4) The perspective view of structures for these phases is illustrated in Fig. 1 together with that for pure V 2 O 5 . 1–4) With increasing Li content, nonmagnetic V 5þ ions are converted to magnetic V 4þ ions (S ¼ 1=2), following the increase in conductive electrons. This produces the differ- ence in electronic and magnetic behaviors in each phase. In the metastable "- and -phase, there is one kind of vanadium site and so V 4þ ions have no preferential site. However, in the -phase, there are two vanadium sites that form two kinds of zigzag chains; one is V 4þ O 5 pyramids chain and another is V 5þ O 5 chain. Isobe and Ueda 5) reported that the magnetic susceptibilities of powder -LiV 2 O 5 show good fits to the equations for a S ¼ 1=2 one-dimensional Heisenberg antiferromagnetic linear chain model. In the monoclinic -phase, furthermore, there are three different vanadium sites forming a three-dimensional tunnel structure. It was reported that the stoichiometric bronze (Li 0:33 V 2 O 5 ) shows metal–insulator transition (MI transition) at 180 K. 6) On the other hand, Fig. 2 shows the temperature depend- ence of dc conductivity over the temperature range of 300 – 450 K in Li x V 2 O 5 with 0:4 5 x 5 1:4. 7) The above room temperature is found to increase with increasing lithium content. The in each sample increases with increasing temperature, indicating the thermally activated form ¼ 0 expðE=k B T Þ with the activation energy E and the Boltzmann constant k B . The estimated value of E decreases with increasing lithium constant; 0.31 eV at x ¼ 0:4 and 0.16 eV at x ¼ 1:4. This activation energy is the measure of activation energy required for both electronic and ionic conduction. These activation energies are different from those determined from NMR measurements which give the values for ionic conduction. E-mail: dai@pm.tokushima-u.ac.jp Journal of the Physical Society of Japan Vol. 77, No. 2, February, 2008, 024602 #2008 The Physical Society of Japan 024602-1