| Lithium orthor-phosphate formulated with LiM2(PO4)3(M=Ti, Ge, Zr, Hf) has been studied for a long time as sodium type super ionic conductor (NASICON). This sort of NASICON was recognized as a promising solid electrolyte at the beginning and is gradually used in rechargeable lithium battery and other electrochemical devices. A shortcoming of these materials, which often occur on using, is the quadrivalence metal ion such as Ti4+ is reduced during the intercalation and deintercalation of Li+. The reduction is accompanied with electronic conduction, which then interferes Li+ conduction. In recent years, Li3Sc2(PO4)3 was reported as single lithium ionic conductor, in which no metallic element can be reduced in Li+ intercalation and deintercalation and consequently no electron migration during ion conduction. However, the ionic conductivity Li3Sc2(PO4)3 is relatively low. On the basis of the two sorts of solid electrolytes, Li3-2x(Sc1-xTix)2(P04)3 has been proposed with the hope of the combination of the good properties of both materials. Single and high Li+ conductivity of Li3Sc2(PO4)3 has been reported both in monolithic form or in composites with polymers.In this thesis, a new family of othor-phosphate electrolytes were prepared by using trivalence Al and Ga elements which are cheaper in the first consideration to replace Sc in Li3Sc2(PO4)3. The materials are formulated with Li3-2x(Al1-xTix)2(PO4)3and Li3-2x(Ga1-xTix)2(PO4)3, (x = 1, 0.95, 0.90,......0.60), respectively. Investigationis focused on the solid reaction and sintering process, phase composition and ionic conductivity.The introduction of lithium ion in the materials was realized by using Li3PO4 and Li2CO3 as starting materials, respectively. The solid reactions of the two starting materials with TiO2 (NH4)H2PO4 and Al2O3 or Ga2O3 were performed at 300℃/2hrs and 1100℃/10hrs. The powder products were identified to be LiTi2(PO4)3 type single phase until x≤0.70. Beyond that, an unknown trivial phase was found.Sintering of LiTi2(PO4)3 (x=l) powder obtained after the solid reaction is difficult, and dense ceramic like parts were scarcely obtained. The ionic conductivitymeasurements on some dense LiTi2(PO4)3 ceramics showed that the conductivity was 2.0x10-6Scm-1 at 298K and l.9 x l0-4Scm-1 at 473K.The substitution of A13+ or Ga3+ for Ti4+ largely promoted the sintering of the reactant powders. Almost full dense ceramics with the specific gravity greater than 2.7 g/cm3 were obtained after conventional sintering. The sintering bought about no phase changes in the two systems, keeping single phase of LiTi2(P04)3 until x 0.70. The intensities of the unknown trivial phases were reduced considerably even x 0.70. As compared with Li3-2x(Al1-xTix)2(PO4)3, all peaks of Li3-2x(Ga1-xTix)2(PO4)3 all shift to right by 0.3 degree 20, indicating a even crystalline plane spacing changes due to different atomic radius of A13+ or Ga3+.The ionic conductivity of Li3-2x(M1-xTix)2(PO4)3 (M = Al, Ga) ceramics are obviously enhanced because of the doping of A13+ or Ga3+ in LiTi2(PO4)3. The ionic conductivities reach their maximums as x=0.85 in both Li3-2x(Al1-xTix)2(PO4)3 and Li3-2x(Ga1-xTix)2(PO4)3, which are 2.3 x l0-4Scm-1 and l.9xl0-4Scm-1, respectively. The enhancement in conductivity is attributed to the high densification and high concentration of Li+ at the grain boundaries. Li3-2x(Al1-xTix)2(PO4)3 and Li3-2x(Ga1-xTix)2(PO4)3 are certainly regarded as satisfactory solid electrolyte materials possibly used in lithium battery. |
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