Multi-electron transfer of molybdenum/vanadium phosphates as cathodes for lithium-ion batteries

Current intercalation cathodes in lithium-ion batteries have typical 100--200 mAh/g specific capacity in 3--5 V voltage range. To pursue higher energy density, one strategy is to increase specific capacity with more than one-electron transfer per redox center. Calculations suggest that molybdenum and vanadium are the only two multiple-valent elements which can possibly enable two or more electron transfer within the acceptable voltage range (3 - 4.5 V) in phosphates. This thesis tries to prove the high possibility of reversible multiple electron transfer in cathodes, which enlarge the scope of current batteries.;First, two molybdenum phosphates cathode materials have been synthesized, layered delta-(MoO2)2P2O7 and novel tunnel structure Li3(MoO)4(PO4) 5. delta-(MoO2)2P2O7 can reversibly intercalate almost 1 lithium ion (per Mo) with a capacity about 110 mAh/g when cycled between 2.3 - 4 V and 70 mAh/g in 2.75 - 4.0 V. However, the structure becomes amorphous when 2 lithium ions are intercalated. The Li3(MoO)4(PO4)5 can reversibly intercalate more than one lithium ion (per Mo) for up to 20 cycles. The structure of this novel compound has been solved from single crystal pattern.;In addition to the molybdenum phosphate, we have also studied VOPO 4 as a possible two-electron candidate for lithium ion batteries. &egr;-LiVOPO 4 has been successfully obtained by solid state method. The compound has V4+/V5+ and V3+/V 4+ redox couples at around 3.8 and 2.5 V, respectively. Despite its high theoretical capacity, the material has intrinsic poor ionic and electronic conductivity that may hinder its electrochemical performance. To overcome these two challenges, smaller particle size with conductive coating is employed. The as-optimized &egr;-LiVOPO4 can reversibly intercalate 1.6 lithium for 20 cycles with capacity of over 240 mAh/g at C/20. Moreover, the delithiated phase &egr;-VOPO4 from hydrothermal precursor give lower practical capacity compared to the optimized &egr;-LiVOPO4. Our results show that its electrochemical performance can be improved by substitution of up to 5 % Mo into its structure, resulting into: enhanced initial capacity from 200 mAh/g to 250 mAh/g (~1.6 Li), better capacity retention of about ~80 % for up to 20 cycles at C/25 and faster kinetics with the reduced voltage hysteresis.