Enhancement of electrochemical performance of Al-doped LiVPO4F using AlF3 as aluminum source

Abstract

Elemental doping is one of the most effective strategies to improve the electrochemical performance of electrode materials for lithium ion batteries. The Al-doped LiV1−xAlxPO4F (x=0, 0.01, 0.02, named as AF-0, AF-1, AF-2, respectively) cathode materials are prepared by a soft chemical lithiation process using AlF3 as aluminum source. For comparison, LiV0.99Al0.01PO4F sample is also synthesized by using Al2O3 as Al precursor (named as AO-1). The X-ray diffraction analysis shows that parts of V sites are substituted by Al atoms, without changing the primary triclinic structure of LiVPO4F. A higher purified LiV0.99Al0.01PO4F sample with less monoclinic Li3V2(PO4)3 is obtained by using AlF3 as aluminum source than that prepared by employing Al2O3. The charge–discharge tests show that AF-0, AF-1, AF-2, AO-1 can deliver specific discharge capacity of 132, 131, 132, 127mAhg−1, respectively. The AF-1 sample possesses the best cyclic performance, maintaining the capacity retention of 96.9% after 50 cycles at 0.5C rate. Although the capacity decreases for all samples as the current density increasing, the Al-doped samples show less discount than the undoped one. Furthermore, AF-1 performs the best electrochemical performance, delivering 118, 98mAhg−1 at 3, 6C, respectively. CV tests indicate that the as-prepared Al-doped samples show lower polarization and higher discharge potential plateaus than the pristine one, and the AF-1 functions better than AO-1.