Determination of the chemical diffusion coefficient of Li + in intercalation-type Li 3 V 2 (PO 4 ) 3 anode material X.H. Rui, N. Yesibolati, S.R. Li, C.C. Yuan, C.H. Chen ⁎ CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Anhui Hefei 230026, China abstract article info Article history: Received 25 November 2010 Received in revised form 15 February 2011 Accepted 16 February 2011 Available online 10 March 2011 Keywords: Lithium vanadium phosphate Lithium ion battery Anode Chemical diffusion coefficient The chemical diffusion coefficients of lithium ion (D Li +) in intercalation-type Li 3 V 2 (PO 4 ) 3 (LVP) anode material as a function of cell voltage between 3.0 and 0.0 V are systematically determined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT). The true chemical diffusion coefficients (D Li + true ) obtained from EIS and GITT for the single- phase region (1.6–0.0 V vs. Li + /Li) are in the range of 10 -10 to 10 -9 cm 2 s -1 and 10 -11 to 10 -10 cm 2 s -1 , respectively, and exhibit a decreasing trend of the change of D Li + true vs. voltage as the discharge and charge proceeded. The apparent chemical diffusion coefficients (D Li + app ) measured from CV and GITT for the two-phase regions (around 2.5–1.6 V) are in the range of 10 -10 cm 2 s -1 and 10 -12 to 10 -10 cm 2 s -1 , respectively. For GITT, D Li + app vs. voltage plots display a characteristic of “W” shape due to the strong interactions of Li + with surrounding ions. Finally, the D Li + values of LVP anode are compared with other anode materials, illustrating that LVP can also be used as a potential anode material to achieve high rate capability. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The monoclinic lithium vanadium phosphate, Li 3 V 2 (PO 4 ) 3 (LVP), has been employed as a promising cathode material in rechargeable lithium ion batteries, due to its good ionic mobility, high reversible capacity and relatively high operating voltage [1–3]. The 3D frame- work structure of LVP is built from slightly distorted VO 6 octahedra and PO 4 3- tetrahedral anions [4]. Each VO 6 octahedron is surrounded by six PO 4 tetrahedra, whereas each PO 4 tetrahedron is surrounded by four VO 6 octahedra. This configuration containing relatively large interconnected interstitial space is potentially a fast ionic conductor. One of the major problems limiting the application of LVP cathode in high power density batteries is its low intrinsic electronic conductiv- ity (about 10 -8 Scm -1 [5]). As a result, many investigations have been focused on its synthesis and improvement of electrochemical capacities in non-queous electrolytes [6–8]. In the past few years, conductivity is usually enhanced appreciably by coating LVP particles with electrically conductive carbon materials [6–8]. Interestingly, in our recent work, we have successfully confirmed that the carbon coated Li 3 V 2 (PO 4 ) 3 (LVP/C) can also be used as an intercalation-type anode material with superior electrochemical per- formance [9]. It exhibits a stable reversible capacity of 203 mAh g -1 in the voltage range of 3.0–0.0 V vs. Li + /Li, and the Coulombic efficiency is close to 100% after the third cycle. In addition, the monoclinic LVP structure can be still retained during the lithium ions insertion/ extraction process. In the discharge process, there are initially a sequence of two-phase transition processes taking place at 1.95 V (Li 3 V 2 (PO 4 ) 3 →Li 3.5 V 2 (PO 4 ) 3 ), 1.86 V (Li 3.5 V 2 (PO 4 ) 3 →Li 4 V 2 (PO 4 ) 3 ), 1.74 V (Li 4 V 2 (PO 4 ) 3 →Li 4.5 V 2 (PO 4 ) 3 ) and 1.66 V (Li 4.5 V 2 (PO 4 ) 3 →Li 5 V 2 (PO 4 ) 3 ), respectively. Subsequently, a single-phase region between 1.6 V and 0.0 V is occurred, corresponding to 2 Li + insertion (Li 5 V 2 (PO 4 ) 3 →Li 7 V 2 (PO 4 ) 3 ) associated with the V 2+ /V + redox couple. And the lithium ions extraction is a reversible process. Furthermore, with increasing interest in high power density of Li-ion batteries, kinetics of lithium ions transfer in LVP anode material is also necessary to be evaluated and understood since they govern the intercalation/deinter- calation rate. In literatures, several techniques including cyclic voltammetry (CV) [10,11], electrochemical impedance spectroscopy (EIS) [12,13] and galvanostatic intermittent titration technique (GITT) [14,15], have been extensively used to study the diffusion kinetics of Li + intercalation/ deintercalation and to estimate the chemical diffusion coefficients of Li + in solid electrodes. Here, it should be noted that, for the diffusion coefficient with a physical meaning (named true diffusion coefficient, D Li + true ), the Li + concentration of electrodes must be changed mono- tonically as intercalation proceeds [16], which is valid only for topotactic solid-state intercalation reactions. When the intercalation of lithium ions is accompanied by strong electron–ion interactions, the intercalation proceeds following one or several reaction fronts, and leads to the coexistence of two phases [15]. Thus, the physical meaning of the chemical diffusion coefficient, D Li +, as a function of lithium composition becomes obscure. However, the obtained D Li + can be taken Solid State Ionics 187 (2011) 58–63 ⁎ Corresponding author: Tel.: + 86 551 3606971; fax: + 86 551 3601592. E-mail address: cchchen@ustc.edu.cn (C.H. Chen). 0167-2738/$ – see front matter © 2011 Elsevier B.V. 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