Reversible Formation and Decomposition of LiF Clusters Using Transition Metal Fluorides as Precursors and Their Application in Rechargeable Li Batteries

Abstract

Whether a solid material can take part in an electrochemical reaction is, besides thermodynamic constraints, determined by kinetic factors, such as the rate of charge transfer and transport within the bulk and across the interface. In view of low electronic and ionic conductivity, Li 2 O has been regarded as an electrochemical inactive material at room temperature. However, recent studies of Tarascon's group show that if Li 2 O is dispersed with transition metal elements (M = Fe, Co, Ni, Cu) on a nanoscale, Li can be electrochemically extracted from M/Li 2 O nanocomposites. [1,2] This surprising and technologically highly relevant finding may be due to the reduced transport pathways, the interface chemistry and/ or size effects on these properties. [3,4] It is known that discharging a Li/MO x cell can transform the metal oxide in situ into a M/Li 2 O nanocomposite.[5±6] In this way, Li can be reversibly stored in lithium batteries by forming and decomposing Li 2 O using transition metal oxides as precursors: [1,2,7,8] MOIn view of technological application, it is significant to note that the nanocomposite need not be pre-fabricated but forms during the insertion of lithium into the transition metal oxide.Like Li 2 O, LiF is also known as a poor electronic and ionic conductor at room temperature, [9] and particularly strong bonds are formed between lithium and fluorine (the electronegativity difference between lithium and fluorine is 3, compared to 2.5 for Li 2 O). Hence it has never been reported to be electrochemically active in a Li cell. It is naturally very relevant to find out whether transition metal fluorides can show a similar electrochemical behavior and whether the very stable LiF can be reversibly decomposed if dispersed with transition metals on an atomic or nanometer scale.The electrochemical reaction of metal fluorides with Li and its reverse reaction may be written as:The emf values of above reactions are calculated for various metal fluorides (M = Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Ag, Sn) according to Nernst equation (interfacial energy has been neglected) from thermodynamic data [10] and listed in Table 1.All the metal fluorides considered here naturally exhibit a lower reaction voltage than the decomposition voltage of 6.1 V for pure LiF, but higher values, by about 1 V, than the corresponding metal oxides. In view of thermodynamics, it is still possible to investigate the above reactions in common nonaqueous electrolyte systems with an electrochemical window of a width of 5 V. In addition, according to Table 1, most transition metal fluorides show theoretical gravimetric capacities which are comparable to the transition metal oxides, but much higher than for graphite (< 372 mA h g ±1 ), which has been used as anode material in commercial Li ion batteries.Hence, we found it relevant to investigate the electrochemical reactions of MF n (M = Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Ag, Sn; n = 2 or 3) with lithium at room temperature. In this communication, we report mainly the results of TiF 3 and VF 3 in vie...

Space Charge Layer Effect in Solid State Ion Conductors and Lithium Batteries: Principle and Perspective

“…(7) yields a potential profile (11) and the defect concentration profile becomes (12) As the mobile majority carrier is depleted, MottSchottky analysis mainly applies to resistive boundaries. If however a minority carriers of the same sign as the impurity has a very high mobility, it can well be conceived that its accumulation leads to a conductance increase even though the impurity level is not exceeded.…”

“…Variation of not only the magnitude but also the type of conductivity were verified in composites, grain boundaries in polycrystalline materials and epitaxial heterostructures. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Conductivities of various functional materials might be manipulated for fundamental research as well as application. Introduction of interfaces not only led to strong variations in conductivity, but also induced qualitatively change of the type of conductivity.…”

Iron(III) fluoride synthesized by a fluorolysis method and its electrochemical properties as a positive electrode material for lithium secondary batteries

“…A new positive electrode material with large capacity is needed for these devices, because current positive electrode materials utilize insertion reactions with intrinsically limited capacities based on one-(or less) electron reaction per formula unit (140 mAh g −1 for LiCoO 2 (0.5 Li) [4,5] and 170 mAh g −1 for LiFePO 4 (1 Li) [6]). Thus, instead of such insertion materials, iron(III) fluoride (FeF 3 ) has been receiving attention as a positive electrode material with a high theoretical capacity of 712 mAh g −1 based on the three-electron reaction, reasonably high average operating potential of 2.7 V vs. Li + /Li, in addition to abundant resources of iron [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26].…”

Transport of External Lithium Along Phase Boundary in LiF-Ti Nanocomposite Thin Films

“…Besides the conversion reaction, it is also noticed that extra lithium can be stored reversibly at the low voltage range after the formation of LiF-Ti nanocomposite. 14 Jamnik and Maier explained such heterogeneous storage phenomenon as the interfacial charging. 15,16 In previous reports on lithium storage through conversion reaciton, the nonaqueous electrolyte was used in the test cells.…”

“…15,16 In previous reports on lithium storage through conversion reaciton, the nonaqueous electrolyte was used in the test cells. 14,18 Consequently, the formation of the solid electrolyte interphase (SEI) is not avoidable. Therefore, the interfacial charging and the SEI evolve within the similar low voltage range, at least for the initial charge-discharge cycles.…”