Effect of chemical additives on the performance of Na/NiCl2 cells

Abstract

Abstract. The effect of chemical additives on the performance of sodium/nickel chloride cells was investigated in quasi-sealed laboratory research cells. The performance of these ceils was measured by galvanostatic and galvanodynamic methods. It was observed that the use of sodium bromide, sulfur, sodium iodide, and a combination of these additives enhance the performance of the Na/NiC12 cells by reducing the area-specific impedance of the nickel chloride electrode. Improved morphology by the use of the poreformer further improves the nickel utilization and the electrode impedance. The performance enhancement is attributed to the chemical and morphological modifications of the nickel chloride electrode in the Na/NiC12 cells. 1, IntroductionThe high theoretical specific energy (790 Wh/kg), high cell voltage (2.59 V), wide temperature range, cell failure mode, and good cycling characteristics of sodium/-nickel chloride (Na/NiC12) battery system have made it a viable candidate for electric vehicle propulsion [1][2][3][4][5][6][7][8][9][10][11][12].The Na/NiCL cell is similar to the Na/S cell in that it uses much of the same technology. For instance, the Na/NiC12 cell, like the Na/S cell, uses a liquid sodium negative electrode and the ]3"-alumina solid electrolyte.However, unlike the sodium/sulfur cell, it uses a secondary electrolyte of molten sodium tetrachloroaluminate (NaA1Cla) in the positive electrode and an insoluble nickel chloride as the active material. The NaAICI4 electrolyte conducts sodium ions from the I]"-A1203 electrolyte to the nickel chloride electrode reaction site. Sodium and Na[A1C14] are molten at the operational temperature (170 to 400 ~ of the cell. Na[A1C14] is added to the porous Ni/NiCI 2 electrode to transport Na + ions from the surface of the [3"-alumina electrolyte to the reaction sites at the interior of the positive electrode. The overall cell reaction is 2Na + NiC12 ~ 2NaC1 + Ni The positive electrode is fabricated from a mixture of NaC1 and nickel powder in the discharged state. During the initial charge, nickel reacts with the chloride ions to form a porous nickel chloride electrode while the sodium ions being reduced at the negative electrode and forming sodium electrode. The electrochemical behavior of NiC12 as the positive electrode in the Na/NiCI 2 cells has been extensively studied [7][8][9][10][11][12]. Most of the previous studies [2,[8][9][10][11] acknowledge that the cell performance is limited due to the NiC12 electrode. A detailed study calried out [10,11] on nonporous nickel electrodes [10] suggested that a low-conductivity nickel chloride (NiC12) layer formed on the electrode during charge. The increasingly higher resistance of the charge product inhibits further thickening of the layer and thereby limits the charge uptake and, consequently, the available capacity in the subsequent discharge. Modifying the electrolyte with the NaBr, NaI and sulfur additives was found to produce higher nickel utilization and lower impedance values due to doping effects which is believed...

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