## 1 Introduction  
Lithium ion batteries are suitable power sources for newly developed mobile devices (cellphones, laptops, cameras, etc.) because of their superior electrochemical characteristics in comparison with classical batteries (Ni-Cd, Ni-MH). Lithium ion batteries possess high energy density, improved life cycling performance, and higher working potential. Cathode materials are one of the key components of Li-ion batteries, and can determine capacity and power performance. Olivine-type lithium iron phosphate material is a very promising cathode material for lithium-ion batteries due to its high theoretical capacity (170 mAh/g) and it has an average charge/discharge voltage of 3.4 V [1]. In addition, the material is quite cheap and it is environmentally friendly, with good thermal and cycling stability. The main disadvantages are low electronic and ionic conductivity because of the slow rate of lithium ion diffusion through the material's solid structure [2]. In general, the following reaction (1) describes the LiFePO\({}_{4}\) cathode material's behavior in a lithium-ion battery:  
\[\mathrm{LiFePO_{4}}-xe^{-}\rightleftarrows(1-x)\mathrm{LiFePO_{4}}+x\mathrm{ FePO_{4}}+x\mathrm{Li}^{+} \tag{1}\]  
This process in the forward direction can be carried out chemically in solution, using the Ox-Red reaction. In this case, the chemical reaction of LiFePO\({}_{4}\) oxidation can proceed either through a solid-phase mechanism (1) or with the dissolution of LiFePO\({}_{4}\) crystals. Dissolution occurs in acidic aqueous solutions (pH < 4), presumably due to the replacement of lithium ions in LiFePO\({}_{4}\) with hydrogen ions to form a soluble hydrogen phosphate. In particular, the ability of LiFePO\({}_{4}\) dissolution in acidic media helps to determine the mass fraction of carbon [3]. Usually, for the dissolution of lithium iron phosphate, a mixture of sulfuric (50%) and hydrochloric (50%) acids (1:1) is used while heating and stirring the solution. Under these conditions, insoluble carbon remains as a precipitate which can be filtered out, dried, and gravimetrically determined. The dissolution of LiFePO\({}_{4}\) in an acidic medium can be used to create an easy and fast method for determining the degree of oxidation of this material (\(w\left(\mathrm{Fe}^{3+}\right)=\frac{m\left(\mathrm{Fe}^{3+}\right)}{m\left( \mathrm{Fe}^{2+}\right)}\)). Determination of the degree of oxidation of LiFePO\({}_{4}\) cathode material is an urgent task, because this material is fairly active and can be easily oxidized from the surface during storage in an oxygen atmosphere. In addition, there are a number of synthesis methods where trivalent iron compounds are used as precursors [4; 5], whereas in certain synthesis conditions, the presence of Li\({}^{+}\) and PO\({}_{4}\)\({}^{3-}\) ions should lead to LiFePO\({}_{4}\) formation. Controlling the iron oxidation state during synthesis is very important, because if Fe\({}^{3+}\) ions remain into the compound, they will decrease the mass of the active material (LiFePO\({}_{4}\)) and will prevent reversible interaction/deintercalation of lithium ions during battery cycling. For example, even in the hydrothermal synthesis method [6] where the divalent iron precursor (FeSO\({}_{4}\)) is used, in the absence of a reducing agent (ascorbic acid), LiFePO\({}_{4}\) with a Fe\({}^{3+}\) impurity is obtained. The obtained LiFePO\({}_{4}\) is significantly lower in electrochemical characteristics with respect to LiFePO\({}_{4}\) obtained by the same method in the presence of an inert atmosphere or reducing agent. In a neutral or alkaline medium, this material is insoluble. Using the chemical reaction of oxidation in a non-acidic medium, it is possible to determine the kinetic parameters of reaction (1), such as activation energy and diffusion coefficient [7]. Moreover, the obtained transport parameters will characterize the cathode material itself directly, and not the total characteristics of the composite electrode (binder, electroconductive additive, active material).