Transition metal phosphates such as LiFePO4 and LiCoPO4 have attracted ... Displacive structural ... been reported for reconstructive phase transitions [9, 10].
Li4VO(PO4)2 : SYNTHESIS, STRUCTURE AND ELECTROCHEMICAL BEHAVIOR M. DUBARRY1, J. GAUBICHER1, G. WALLEZ2 , P. MOREAU 1 M. MORCRETTE3 , M. QUARTON2 AND D. GUYOMARD1 1
Institut des Matériaux Jean Rouxel, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France 2 Laboratoire de Cristallochimie du Solide, Université Pierre et Marie Curie-Paris VI, 4 place Jussieu, 75252 Paris Cedex 05, France 3 Laboratoire de Réactivité et de Chimie des Solides, 33 rue Saint Leu, 80039 Amiens, France
ABSTRACT The electrochemical investigation of a new vanadium phosphate, Li4VO(PO4)2, has been undertaken. Starting from pristine VO(H2PO4)2, Li2VO(HPO4)2 and Li4VO(PO4)2 compounds are obtained through an ionic exchange. The system Li/Li4VO(PO4)2 shows several phase transitions that are associated to potential hysteresis whose values vary from roughly 5 mV to more than 1.8 V. Ex situ and in situ structural studies during cycling allow the determination of the origin of the hysteresis and better understanding of the different redox processes appearing along the reduction or oxidation of this compound.
INTRODUCTION Transition metal phosphates such as LiFePO4 and LiCoPO4 have attracted great interest as new cathode materials for lithium rechargeable batteries [1-3]. In the continuing search for attractive polyanion insertion hosts, various structures of vanadyl phosphate, VOPO4, have also received recent attention [4-8]. Lithium electrochemical intercalation-deintercalation in host structures results most of the time in phase transitions that are either 1st or 2nd order. Displacive structural transitions are associated to small hysteresis, whereas hysteresis larger than 0.2V have been reported for reconstructive phase transitions [9, 10]. This study concerns a new vanadium phosphate, Li4VO(PO4)2, that exhibits first order structural phase transitions associated to hysteresis values that span from a few mV to more than 1.8V.
EXPERIMENTAL Li4VO(PO4)2 is obtained from VO(H2PO4)2 upon a H+/Li+ ionic exchange in various non aqueous solutions (Li-Ethanolate, LiOH,H2O in a mixture of absolute ethanol and acetone). The pristine VO(H2PO4)2 is synthesised upon reduction of V2O5 powder in
aqueous solution of H3PO4 and HBr at 100°C for 12h under constant argon flow. Interestingly the H+/Li+ ionic exchange using LiOH,H2O dissolved in a mixture of acetone and absolute ethanol has been reported to be impossible whereas it leads to Li4VO(AsO4)2 for the analogous arseniate [11-14]. Our results show that this ionic exchange is readily possible and leads to Li4VO(PO4)2. XRD data were collected with a Θ/2Θ SIEMENS D5000 diffractometer with a linear MOXTEK detector and on an INEL diffractometer with CPS detector. SEM images were obtained from a GEOL 6400 microscope. Electrochemical measurements were achieved with Mac-Pile or VMP controllers using Swagelok type cells. The composite electrode was coated on an Al disc from a mix of 80% of Li4VO(PO4)2, 15% of carbon black (Super P) and 5% of PVdF. EC:DMC(2:1) + 1M LiPF6 (from MERCK) was used as the electrolyte.
RESULTS & DISCUSSION VO(H2PO4)2 crystallises in a quadratic cell with P4/ncc space group (Figure 1) [15,16]. This compound exhibits a layered structure, each layer consisting of VO5 squarebased pyramids linked to four identical VO5 neighboring polyhedra by the base through PO4 tetrahedra. Each phosphate group links two VO5 units and the remaining two oxygen atoms are protonated. Successive layers are held together by VO-VO interactions and hydrogen bonding between a protonated oxygen from one layer and two protons in the other one.
Figure 1 : Structure of VO(H2PO4)2
XRD results show that the exchange of Li+ for H+ in the pristine compound proceeds via the formation of Li2VO(HPO4)2 and leads finally to Li4VO(PO4)2. The three phases (VO(H2PO4)2, Li2VO(HPO4)2, Li4VO(PO4)2) show a small non stoechiometry domain with respect to H+/Li+ composition. From XRD study it appears that the frameworks of Li2VO(HPO4)2 and Li4VO(PO4)2 are isostructural with that of the pristine VO(H2PO4)2.
The total exchange of Li+ for H+ ions results in variation of the cell parameters of about -1.5% and + 10% for a and c respectively. That corresponds to a slight contraction within the layers and a large increase of the interlayer space. It is associated to a breaking of VO(H2PO4)2 grains (rods of 10*1*1µm) presumably along the c axis, that gives parallelepiped grains of approximately 1µm3 (figure 2).
⇒ VO(H2PO4)2
Li4VO(PO4)2
Figure 2 : SEM morphology of pristine VO(H2PO4)2 and final Li4VO(PO4)2
Electrochemical lithium insertion/deinsertion is reported on figure 3 using voltametric cycling (+/- 5mV) with galvanostatic acceleration (Imin ≈ 1Li/25h) the electrochemical behavior could be described with 2 redox areas :
T2aox
V3+/V4+
V4+/V5+
B Li5
B T2bred
T2box
A T2ared
S1ox
Li4
A S1red
T1ox S2ox
Li3
S2red
T1red
Figure 3: Potentiodynamic cycling (+/- 5mV, Ilim=1Li/25h) starting on oxidation
Oxidation of the V4+ ion to V5+ ion leads to the phase “Li3VO(PO4)2 ” within 3 steps. The S1 and S2 peaks are large and characteristic of a Nernstian process, related to a non stoechiometric domain. The relative position of the oxidation and reduction peaks associated to T1 are characteristic of a two-phase process with equilibrium potential close to 4.14V. The potential hysteresis value is about 5mV. Reduction of V4+ to V3+ starts from 2.04V. It occurs in two close phenomena : one sharp peak that starts from 2.03V (noted T2ared ) and T2bred that appears as a shoulder. It corresponds to the insertion of 0.4 and 0.6 Li respectively. The corresponding capacity is fully recovered upon oxidation. A small fraction of it is obtained in the 2V region whereas the main part occurs from 3.84V on a very sharp peak. The reduction limit potential has been stopped either on 2.04V (A) before T2a starts, at 2V during T2a process
or at 1.5V (B) upon T2b. It appears that the sharp peak at 3.8V occurs only if T2a red is performed. The oxidation peak at 3.8V was thus assigned to T2ared and noted T2aox and the one at 2V to T2b as T2box. According to these observations, the hysteresis value related to the T2a transition is close to 1.8V. In order to understand the different phenomena in-situ XRD measurement was made during 1 cycle. The measurement was performed anode in LRCS (Amiens, France) with a PSD diffractometer with cobalt using an home made in-situ cell with a beryllium window. The rate of the experiment was of 1 lithium per formula unit in 10 hours for the first reduction and the total oxidation and of 1 lithium in 5 hours for the return to the initial potential (Figure 4). The XRD patterns was made in 30 minutes during OCV. Moreover, to determine the influence of structure on the hysteresis values encountered in this system, ex situ diagrams have been collected after completion of equilibrium at several potential values.
3.5V 4.5V
T1ox 1.5V
T2bred T2ared Initial
3.5V 2Θ Co kα Figure 4 : In-situ XRD measurements
From Full Pattern Matching refinements of the collected XRD data a better understanding of each redox area is achieved : V4+/V5+ : In-situ XRD studies confirm the nature of S1, T1 and S2 processes : 0.4 Li are deintercalated from Li4VO(PO4)2 through S1. At this point, a first order phase transition occurs between Li3.6VO(PO4)2 and Li3.15VO(PO4)2 compositions which drives to Li3VO(PO4)2 through S2. The phase Li3+xVO(PO4)2 (0
