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.
Thermodynamics of the precipitation from Li-Fe（II）-P-H2O system at 298 K was investigated.The results demonstrate that LiFePO4 can be formed at room temperature under pH value of 0-11.3,and the impurities Li3PO4 and Fe（OH）2 will be yielded at pH value above 11.3 and 12.9,respectively.The optimum pH value for LiFePO4 precipitation is 8-10.5.Considering the low rate of phase transformation kinetics,metastable Li-Fe（II）-P-H2O system was also studied.The results indicate that equimolar ratio of co-precipitation precursor Fe3（PO4）2.8H2O and Li3PO4 cannot be obtained at the initial molar ratio 1：1：1 and 1：1：3 of Li：Fe：P.In contrast,equimolar ratio of the co-precipitation precursor can be yielded by adjusting the pH value to 7-9.2,matching the molar ratio 3：1：1 of Li：Fe：P,meaning that Li＋-excess is one of the essential conditions for LiFePO4 preparation by co-precipitation method.
