Electrochemical behavior of Li+, Mg2+, Na+ and K+ in LiFePO4/ FePO4 structures

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.