## 3 Results and Discussion
### Chemical Oxidation in Acidic Medium  
Unlike alkaline solutions, LiFePO\({}_{4}\) dissolves into Li\({}^{+}\), Fe\({}^{2+}\), and PO\({}_{4}\)\({}^{3-}\) ions in strong acidic medium. This is very convenient for determining the degree of oxidation of the material, because chemical oxidation in acidic solution allows a homogenous oxidation reaction of Fe\({}^{2+}\) to Fe\({}^{3+}\). In this paper, the oxidation in an acid medium was used to determine the degree of oxidation of the synthesized material. The degree of oxidation of of LiFePO\({}_{4}\) was determined by Ox-Red  
Figure 6: Schematic illustration of the oxidation equipment. 1—magnetic stirrer; 2—platinum electrode; 3—Li selective electrode; 4—pH glass electrode, 5—Ag/AgCl electrode; 6—glass cell; 7—stirrer; 8—ionometer. Figure 7: Kinetic curves of LiFePO\({}_{4}\)/C chemical delithiation process at different temperatures. titration with potassium dichromate of a dissolved sample in an acidic medium. The known mass of LiFePO\({}_{4}\)/C was added to a 50-mL volume of sulfuric acid solution (50%) with stirring, and after complete dissolution, the resulting solution was titrated with 0.05 N potassium dichromate solution. The equivalence point was fixed potentiometrically. The setup circuit for the analysis is similar to the previous scheme (Figure 6), but without the usage of a lithium-selective electrode. LiFePO\({}_{4}\)/C dissolution in an acidic medium can be observed by the change in the Ox-Red potential of solution when an LFP/C powder sample is added into the solution (Figure 8a). The change in the Ox-Red solution potential is due to the formation of a new Ox-Red pair (Fe\({}^{3+}\)/Fe\({}^{2+}\)). The of Fe\({}^{3+}\) ions are the result of the powder's partial oxidation. The complete dissolution of the sample is confirmed by the Ox-Red potential plateau. A suspension of insoluble carbon interferes with the visual determination of the equivalence point during titration with K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\), so fixation of the equivalence point was carried out potentiometrically. When the equivalence point was reached, a sharp jump in the potential was registered due to the formation of a new Ox-Red pair (Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\)/Cr\({}^{3+}\)) (Figure 8b). The established Ox-Red potential of the solution increased insignificantly after the jump, followed a plateau, which suggests the complete oxidation reaction. The oxidation degree of synthesized LiFePO\({}_{4}\)/C according to the results of titration in an acidic medium is 91.5%. This value is reproducible and the average error is only 0.2%. To confirm the efficiency of this method, the synthesized LiFePO\({}_{4}\)/C was half oxidized (\(t\) = 200 s) and completely oxidized (\(t\) = 2000 s), respectively, in alkaline medium by hydrogen peroxide (at room temperature) followed by the filtration and drying of the powder. For the determination of the oxidation degree, the latter obtained samples were analyzed by Mossbauer spectroscopy, XRD, and chemical oxidation method in acidic medium. XRD patterns indicate the presence of two phases (olivine and heterozygite) in the case of half-oxidized material (Figure 9a) and only the heterozygite phase for completely oxidized material (Figure 9b). The oxidation degree of half-oxidized and completely oxidized LFP determined by Mossbauer spectroscopy is 45.2% and 100%, respectively. This is in good agreement with the results of the titrimetric method--4.8% and 100%, respectively. The good correlation between Mossbauer spectroscopy and titrimetric method indicates the validity of the suggested analysis method, but without additional XRD analysis it cannot be affirmed that all titrated Fe\({}^{2+}\) ions are found only in LiFePO\({}_{4}\). The chemical oxidation method in acidic medium is convenient for application in laboratory studies. Better results can be achieved by optimizing LFP synthesis and improving the determination of the carbon content from the sample.