Electrochemical behavior of Li+, Mg2+, Na+ and K+ in LiFePO4/ FePO4 structures
(1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China;
2. Key Laboratory of Hunan Province for Metallurgy and Material Processing of Rare Metals, Changsha 410083, China)
2. Key Laboratory of Hunan Province for Metallurgy and Material Processing of Rare Metals, Changsha 410083, China)
Abstract: Besides Li+ and Mg2+, the electrochemical behavior of Na+ and K+ in LiFePO4/ FePO4 structures was studied since they naturally coexist with Li+ and Mg2+ in brine. The cyclic voltammogram (CV) results indicated that Na+ exhibits some reversibility in LiFePO4/FePO4 structures. Its reduction peak appears at -0.511 V, more negative than that of Li+ (-0.197 V), meaning that a relatively positive potential is beneficial for decreasing Na+ insertion. The reduction peak of K+ could not be found clearly, indicating that K+ is difficult to insert into the FePO4 structure. Furthermore, technical experiments using real brine with a super high Mg/Li ratio (493) at a cell voltage of 0.7V showed that the final extracted capacity of Li+, Mg2+ and Na+ that can be attained in 1 g LiFePO4 is 24.1 mg, 7.32 mg and 4.61 mg, respectively. The Mg/Li ratio can be reduced to 0.30 from 493, and the Na/Li ratio to 0.19 from 16.7, which proves that, even in super high Mg/Li ratio brine, if a cell voltage is appropriately controlled, it is possible to separate Li+ and other impurities effectively.
Key words: LiFePO4/FePO4 electrodes; NaFePO4; lithium; potassium; brine