(Invited) New Insight of NaLiFePO4f As a High Voltage Cathode Material for Lithium-Ion Battery
Authors: Bai Zhao-Wen , Shao Minhua , Chen Guohua 
Published: February 2020

The lithium-ion battery is one of the most promising electrochemical energy storage system having great potential for a wide range of applications. However, there are certain challenges that need to be addressed such as low practical energy density, short lifetime and high price. Three-dimensional polyanion compounds (XO4)m- (X=P, S) with impressing electrochemical performance and stability are considered as the cathode material for next generation lithium-ion battery[1, 2]. Meanwhile, combining high ionic M-F bond with (XO4)m- (X=P, S) is able to enhance the operating voltage, such as LiVPO4F which has already been reported with good performance[3]. Comparing with other fluorophosphates, NaLiFePO4F has received particular interest due to its high operating voltage and the potential of more than one alkali ion can be activated[4, 5]. Meanwhile, several problems are associated with this material such as low electronic conductivity and limited knowledge about the mechanism of the second lithium-ion activation during the charging and discharging process[6]. In order to solve these problems, first principle simulation based on density functional theory, is employed to investigate the mechanism of lithium-ion extraction of NaLiFePO4F (Li2FePO4F). It is found that not only the transition metal Fe2+ but also O2- will lose electron during the delithiation process by electronic structure analysis. Operating voltages and structural evolution are obtained after the calculation of lithiated phases LixFePO4F (x=2, 1.5, 1, 0.5, and 0). The oxidation potentials for different lithium ion concentration in LixFePO4F compounds are located at ~3.41, 3.51, 4.80, and 5.60V respectively. The lithium-ion activation exhibits in the form of a single phase. The NaLiFePO4F is synthesized by annealing the mixture of LiFePO4 and NaF with molar ratio 1:1 at 650ºC for 1.5h. The Li2FePO4F can be obtained by ion-exchange during electrochemical testing. The electrochemical performance is characterized by galvanostatic charge-discharge test between 2~4.5V at rate 0.1C with an average potential~3.4V vs Li+/Li. A reversible specific capacity ~127mAh g-1 can be obtained.

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Figure 1