固态电解质Li_(1+x)Al_(x)Ti_(2-x)(PO_(4))_(3)中Li+的迁移特性

Migration properties of Li^(+)in Li_(1+x)Al_xTi_(2–x)(PO_4)_3

Li_(1+x)Al_(x)Ti_(2-x)(PO_(4))_(3)(LATP)是一种颇具前景的NASICON型锂离子固态电解质.本文通过第一性原理计算研究了不同Al掺杂浓度(x=0.00,0.16,0.33,0.50)对LATP的结构特性、电学特性以及Li^(+)迁移特性的影响.结果表明,Al能够稳定掺杂进入LiTi2(PO4)3(LTP)的晶体结构当中.当Al掺杂浓度x=0.16时,Li—O键的平均键长最长,成键强度最弱,而Ti—O键强度随Al掺杂浓度变化不大.Al掺杂浓度对LATP带隙的影响不大,但Al附近的O原子聚集了更多的负电荷,形成AlO6极化中心.Li^(+)不同的迁移方式(空位迁移、间隙位迁移和协同迁移)在Al掺杂浓度不同时展现出复杂的能垒变化,Li^(+)在空位迁移中迁移势垒随Al掺杂浓度的增大而升高,而在间隙位迁移中Li^(+)的迁移势垒变化相反,由于协同迁移中涉及空位和间隙位两种位点,Li^(+)的迁移势垒表现为随Al掺杂浓度的升高先降低后升高的复杂变化.当x=0.50时,LATP具有最低的Li^(+)迁移势垒0.342 eV,这个势垒值是间隙位迁移的结果.因此,通过改变Al掺杂浓度,可改变间隙Li^(+)浓度及迁移通道结构,进而调节Li^(+)的迁移性能,提高LATP中的Li^(+)导电性能.

NASICON-type materials are specific skeleton structures in which ions move in three dimensions.Li_(1+x)Al_(x)Ti_(2-x)(PO_(4))_(3)(LATP)is a promising NASICON-type solid-state electrolyte for Li-ion batteries,due to its relatively high Li^(+)conductivity,chemical stability to air and moisture,and mechanical strength.Motivated by this,we study the doping and electronic properties of Li_(1+x)Al_(x)Ti_(2-x)(PO_(4))_(3)(x=0.00,0.16,0.33,0.50)and the transport properties of Li^(+)in them by using first-principles calculations based on density functional theory as implemented in Vienna ab initio Simulation Package(VASP).The results indicate that Al can substitute Ti to form a stable structure.When the A1 doping concentration is x=0.16,the average bond length of Li—O bond is longest and the bonding strength is weakest,this may lead to the expansion of channels for Li^(+)migration,which facilitates the diffusion of Li^(+).With the increase of Al doping concentration,the strength of Ti—O bond remains almost unchanged.The electronic structure calculations exhibit that with the increase of Al doping concentration,the bandgap of LATP does not change much,and LATP shows semiconductor characteristic.The differential charge results indicate that more electrons are localized on O-atoms surrounding the Al-dopant,causing the AlO_6 groups to form polarization centers.The study on the migration properties of Li^(+)indicates that Li^(+)exhibits different migration characteristics in three different migration modes(vacancy migration,interstitial migration,and cooperative migration).With the increase of Al doping concentration,the migration barrier of Li^(+)increases via vacancies involving only lattice site migration,and the migration barrier for LATP-0.16 is lowest(0.369 eV).While in interstitial migration involving only interstitial sites,the migration barrier of Li^(+)decreases accordingly.When the Al doping concentration is x=0.50,the migration barrier is lowest(0.342 eV).In terms of cooperative migration,this migration mode involves both vacancy and interstitial sites,so the migration barrier first decreases and then increases with the increase of A1 doping concentration.Thus,our study suggests that by varying the concentration of Al doping,the interstitial Li^(+)content,migration channel structure,and the migration performance of Li^(+)can be changed favorably.Our results provide a theoretical basis for improving the ion conductivity of Li in LATP by varying the Al doping concentration in experiment.