《厦门大学学报（自然科学版）》

采用基于密度泛函理论(DFT)的第一性原理方法研究了钠离子电池正极材料橄榄石型NaFePO4及其碱金属(Li、Na)掺杂体系的结构稳定性、电子结构、充放电过程结构演化及相关电化学性质.计算结果显示,NaFePO4可掺杂适量Li、Na,且掺杂后结构稳定.脱钠相结构形成能的计算模拟了NaFePO4及其掺杂体系充电过程中的相变,NaFePO4在充电过程中存在Na5/6FePO4与Na2/3FePO42个中间相,铁位锂掺杂体系存在Na3/4(Li1/12Fe11/12)PO4、Na1/2(Li1/12Fe11/12)PO4、Na1/6(Li1/12Fe11/12)PO43个稳定中间相,铁位钠掺杂体系存在Na5/12(Na1/12Fe11/12)PO4、Na1/6(Na1/12Fe11/12)PO4、Na1/12(Na1/12Fe11/12)PO43个稳定中间相.此外,通过分析电子结构,还发现Li、Na掺杂体系中有部分O2-发生氧化参与电荷补偿,这使得掺杂体系的充电电压略高于未掺杂的NaFePO4.

In this work,first-principles calculations based on density functional theory(DFT)are performed to study structural stabilities,electronic properties,structural evolution in charge/discharge process and electrochemical properties of olivine NaFePO4 and doped systems with alkali metals of Li and Na for the cathode material of Na-ion batteries.Our results show that the NaFePO4 can be doped with appropriate amounts of Li and Na, and the structure after doping can become stable.The calculated formation energy of the desodiated products simulates the phase transition in the charge process of NaFePO4 and its doping systems.There are two intermediate phases of Na5/6FePO4 and Na2/3FePO4 in the charge process of NaFePO4,as well as three intermediate phases of Na3/4(Li1/12Fe11/12)PO4, Na1/2(Li1/12Fe11/12)PO4, Na1/6(Li1/12Fe11/12)PO4 and of Na5/12(Na1/12Fe11/12)PO4, Na1/6(Na1/12Fe11/12)PO4, Na1/12(Na1/12Fe11/12)PO4 in those doped systems with Li and Na,respectively.In addition,by exploring the electronic properties, it is found that the partial oxygen ions in those doped systems of Li and Na are oxidized to participate in charge compensation,and this mechanism of charge compensation also tends to elevate the voltage of the doped systems to become higher than that of undoped-NaFePO4.

引言
1 计算方法
2 结果与讨论
3 结论

图1 橄榄石型NaFePO4及其掺杂体系的晶体结构<br/>Fig.1 Crystal structures of olivine NaFePO4 and its doped systems

图1 橄榄石型NaFePO4及其掺杂体系的晶体结构
Fig.1 Crystal structures of olivine NaFePO4 and its doped systems

表1 Li、Na掺杂前后橄榄石型NaFePO4的理论晶格常数与橄榄石型NaFePO4的实验晶格常数<br/>Tab.1 Theoretical lattice constants of olivine NaFePO4 before and after Li/Na doping, together with experimental lattice constants of olivine NaFePO4

表1 Li、Na掺杂前后橄榄石型NaFePO4的理论晶格常数与橄榄石型NaFePO4的实验晶格常数
Tab.1 Theoretical lattice constants of olivine NaFePO4 before and after Li/Na doping, together with experimental lattice constants of olivine NaFePO4

图2 Na1-xFePO4、 Na1-x(Li1/12Fe11/12)PO4及Na1-x(Na1/12Fe11/12)PO4的形成能凸包图<br/>Fig.2 Convex hull of calculated formation energies for Na1-xFePO4, Na1-x(Li1/12Fe11/12)PO4 and Na1-x(Na1/12Fe11/12)PO4

图2 Na1-xFePO4、 Na1-x(Li1/12Fe11/12)PO4及Na1-x(Na1/12Fe11/12)PO4的形成能凸包图
Fig.2 Convex hull of calculated formation energies for Na1-xFePO4, Na1-x(Li1/12Fe11/12)PO4 and Na1-x(Na1/12Fe11/12)PO4

图3 Na1-xFePO4、Na1-x(Li1/12Fe11/12)PO4及Na1-x(Na1/12Fe11/12)PO4的平均电压<br/>Fig.3 Average voltages of Na1-xFePO4, Na1-x(Li1/12Fe11/12)PO4and Na1-x(Na1/12Fe11/12)PO4

图3 Na1-xFePO4、Na1-x(Li1/12Fe11/12)PO4及Na1-x(Na1/12Fe11/12)PO4的平均电压
Fig.3 Average voltages of Na1-xFePO4, Na1-x(Li1/12Fe11/12)PO4and Na1-x(Na1/12Fe11/12)PO4

图4 Na1-xFePO4、Na1-x(Li1/12Fe11/12)PO4及Na1-x(Na1/12Fe11/12)PO4的晶胞体积变化率<br/>Fig.4 Volume change rates in Na1-xFePO4, Na1-x(Li1/12Fe11/12)PO4 and Na1-x(Na1/12Fe11/12)PO4unit cell

图4 Na1-xFePO4、Na1-x(Li1/12Fe11/12)PO4及Na1-x(Na1/12Fe11/12)PO4的晶胞体积变化率
Fig.4 Volume change rates in Na1-xFePO4, Na1-x(Li1/12Fe11/12)PO4 and Na1-x(Na1/12Fe11/12)PO4unit cell

图5 计算得到Na1-xFePO4、Na1-x(Li1/12Fe11/12)PO4 及 Na1-x(Na1/12Fe11/12)PO4的PDOS<br/>Fig.5 Calculated PDOS of Na1-xFePO4,Na1-x(Li1/12Fe11/12)PO4 and Na1-x(Na1/12Fe11/12)PO4

图5 计算得到Na1-xFePO4、Na1-x(Li1/12Fe11/12)PO4 及 Na1-x(Na1/12Fe11/12)PO4的PDOS
Fig.5 Calculated PDOS of Na1-xFePO4,Na1-x(Li1/12Fe11/12)PO4 and Na1-x(Na1/12Fe11/12)PO4

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备注

引言

1 计算方法

2 结果与讨论

3 结论

学报简介

备注

引言

1 计算方法

2 结果与讨论

3 结 论

学报简介

3 结论