Thin-Film NASICON-Type Li1+xAlxTi2-x(PO4)3 Solid Electrolyte Directly Fabricated on a Graphite Substrate with a Hydrothermal Method Based on Different Al Sources

A solid electrolyte, aluminum-substituted NASICON-type LiTi2(PO4)(3) (LTP) has been formed and successfully fabricated in situ on a graphite substrate through a hydrothermal approach. The effects of different aluminum sources and addition amounts on Li1+xAlxTi2-x(PO4)(3 )(LATP) characteristics and ionic conductivity have been investigated. The results indicate that Al-2(SO4)(3) is better than other aluminum sources in decreasing the resistance and correspondingly increasing the ionic conductivity. The sample LATP(-Al/Ti-0.2) fabricated with the Al-2(SO4)(3) addition amount of 10 mM has the highest ionic conductivity of 1.65 X 10(-4) S cm(-1), which is significantly higher than other LATP samples. The Li-7 MAS NMR results suggest that LATP(-Al/Ti-0.2) has the highest lithium ion mobility and that, generally, all LATP samples have three kinds of lithium ions located at different interstitial sites. It has been found that LATP(-Al/Ti-0.2) has a relatively higher volume of M-1 interstitial sites and higher c value in crystal parameters compared with other LATP samples, suggesting larger cavities for lithium ions and the weakening of oxygen atom bonding to lithium ions. After sintering treatment to the LATP(-Al/Ti-0.2) sample at 850 degrees C, it shows an even higher total ionic conductivity of 4.80 X 10(-4) S cm(-1) and is applied for the fabrication of a LiFePO4/LATP/graphite cell. The cell exhibits a high discharge capacity of 142.0, 138.1, 131.5, 123.7, and 108.8 mAh g(-1) at different C-rates of 0.05, 0.1, 0.2, 0.5, and 1.0 C, respectively. In general, the preparation of LATP solid state electrolyte film as a fast lithium ion conductor on an anode surface could be a promising new approach for fabrication of all solid state lithium ion batteries (ASSLIBs).