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404 Waste Management & Research 40(4) During the test, the residual voltage percent (Et) in the battery was calculated by comparing voltage remaining after the dis- charge to the initial voltage of the battery. The residual voltage percent was calculated as follows: electrolysis in standard conditions at 25°C is 1.23V, the water degradation potential in the experiments in this paper is affected by the activity of the electrodes and the presence of salt solutions. For example, under particular conditions, decomposition poten- tials of 1.68 V have also been measured (Shen et al., 2011). During electrochemical discharge in aqueous solutions, reac- tions in equations (4) and (5) are responsible for the production and consumption of electrons present in the cell. And, as men- tioned, the battery potential must be higher than the decomposi- tion potential to produce a water splitting reaction. In the case of LIBs, this is easily met, as a fully charged battery usually has an initial potential of about 4V. The more the discharge proceeds, the less potential remains to induce water splitting reactions (Chagnes and Swiatowska, 2015; Lu et al., 2013; Ojanen et al., 2018). It is noteworthy that the voltage in other types of batteries, such as Ni–MH and alkaline batteries are lower at 1.35V and 1.5V, respectively. When sufficient voltage is present across the electrodes in a cell containing NaCl solution in water, it results in the release of H2 gas in one electrode and oxygen or chlorine gas in the other. In other words, at the anode, chloride ions may be oxidized to Cl2. But the Na+ ions in the cathode are not reduced to Na metal. Instead, H2 and OH− ions are produced by reducing the H2O mol- ecules because water is reduced more readily than Na+ ions. This is primarily because the reduction of Na+ results in the produc- tion of highly active sodium metal, whereas the decrease in H2O results in the production of more stable H2 (g) and OH− (aq) products. As salt concentrations increase, the possibility of the partici- pation of various ions rather than just water at the anode and cath- ode becomes more likely. For example, when chloride ions are present in water, the following reaction may occur: Anode(Oxidation):2Cl− →Cl2 +2e− E0 =1.36V (8) Due to the higher potential of reaction in equation (8) compared to reaction in equation (6), water decomposition at the anode seems more likely, but due to the limitations in ion transport and diffusion, the possibility of chlorine gas formation cannot be completely ruled out (Shen et al., 2011; Zumdahl et al., 2018). Chlorine gas is toxic when inhaled, but the amount of chlorine produced in this process is negligibly low. Although in the elec- trolysis process both hydrogen and chlorine gas are produced, but since chlorine gas is more soluble in water than hydrogen, more hydrogen gas is observed in the form of bubbles in this process (Shakhashiri, 1985). Effect of salt type and concentration on discharge The discharge tests for LIBs were conducted with electrolyte solu- tions of NaCl, Na2S, and MgSO4 at different concentrations. These salts were chosen in order to observe the effects of a wide range of molarities and ionic strengths on discharge. Table 1 shows the ionic strength of each solution at a given weight percent. E =Vt *100% tV where Vt is the voltage at time t, and Vo is the initial voltage of the LIB. In addition, we can calculate the percentage of voltage drop (Er) with the following equation (Shakhashiri, 1985): 0 (1) V Er =1−Et =1− t *100% The ionic strength of the salt solutions was calculated with the following equation (Karim and Zahari, 2019; Majlesi et al., 2019): 1∑i=n 2 I=2 cizi i=1 Results and discussion (3) V0 (2) When the anode and cathode of batteries come into contact with water containing salts, the potential difference between the poles leads to electrolysis of the water as the LIB is discharged. During experimentation, this is clearly visible in the form of small bub- bles emerging in the solution when conducting the test. As the discharge process progresses and the battery voltage decreases, the amount of bubbles produced also decreases. This can be justified with the following explanation. Water electrolysis is the breakdown of water molecules into hydrogen gas and oxygen gas using electric force. Electrochemical reac- tions occur at the anode and cathode: Anode: HO()⇔2H++1O2+2e−Eo (25°C)=1.23V (4) 2l 2 anode Cathode: 2H++2e−⇔H E0 2 cathode (25°C)=0V (5) Thus, the complete reaction in the electrolytic cell will be: HO()⇔H +1O E0 (25°C)=1.23V 2l 222cell (6) The Nernst equation expresses the potential of water electrolysis with thermodynamic parameters shown in equation (7). E0 =1.23−0.9×10−3(T −298)+ RT ln H2 cell 4Fp p2 ⋅p O2 (7) H2O where pH2, pO2, and pH2O are the hydrogen, oxygen, and water vapor pressures, respectively. Although the potential of waterPDF Image | Discharge of lithium-ion batteries in salt solutions
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