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Enhanced Electrochemical Performances of Li3V2(PO4)3
Cathode Materials for Li-Ion Battery
Hyun-Soo Kim, Min-Ki Jeon, and Bong-Soo Jin
Battery Research Center, Korea Electrotechnology Research Institute
Changwon 642-120, Korea
[email protected]
Lithium ion batteries have been widely used as a power source from portable
devices to electric vehicles. Cathode materials in the cells are important in determining
the electrochemical properties like operating voltage, energy density, and etc. Currently,
layered LiMO2 materials have been studied as cathode materials for commercialized
LIBs. But, in case of typical layered LiCoO2, it is difficult to applicate in large-scale
batteries due to high cost and toxicity or dissolution of transition metal ions into an
electrolyte at an elevated temperature. On the other hand, polyanion-type materials,
such as LiFePO4, LIFeSO4F, LiMnPO4, LiCoPO4, and Li3V2(PO4)3, etc., possess high
thermal and chemical stability, suggesting candidate materials for large-scale batteries.
Among these materials, Li3V2(PO4)3 (LVP) has a high operating voltage of 4.8V
and a high theoretical capacity of 197mAhg-1. LVP with a NASICON (sodium super
ionic conductor)-type structure enables Li+ ions to diffuse rapidly through a three
dimensional pathway of open structure. In addition, NASICON-type structure are
known as thermally and chemically more stable materials than above materials because
of strong covalent bonding between P and O. However, LVP is not good at high current
rate due to low electric conductivity. The carbon coating is needed to improve electric
conductivity of LVP. Also, various transition metals (Fe, Ti, Mn, and Co, etc.) were
doped to improve electrochemical properties such as capacity, cycleability and rate
capability.
In this study, LVP was synthesized by a microwave-assisted sol-gel method and was
doped with transition metals for improving electrochemical properties. Microwave
heating is process whereby microwaves produced by magnetrons are directed toward
reactants or heating medium, which absorb the electromagnetic energy volumetrically to
achieve self-heating uniformly and rapidly. LVP is synthesized using V2O5, NH4H2PO4,
Li2CO3 and oxalic acid as starting materials, various compounds containing transition
metals, graphene, super P black, and CNT (carbon nano tube) were used for metal
doping and carbon coating, respectively. Oxalic acid and V2O5 in a stoichiometric ratio
are dissolved in deionized water under magnetic stirring. NH4H2PO4, Li2CO3 and
compounds containing transition metals were added when solution is formed of a clear
blue, and the mixed solution was stirred at thermostatic plate to obtain the dark blue gel.
The obtained gel heated at 400℃ for 10min. After grinding, and heated again at 750℃
for 10min under Ar atmosphere. The LVP sample was obtained through the above
experimental procedure.