Effects of Salt Additives to the KOH Electrolyte Used in Ni/MH Batteries

Abstract

KOH-based electrolytes with different salt additives were investigated to reduce their corrosive nature toward Mg/Ni metal hydride alloys used as negative electrodes in nickel metal hydride (Ni/MH) batteries. Alkaline metal halide salts and oxyacid salts were studied as additives to the traditional KOH electrolyte with concentrations varying from 0.005 M to 1.77 M. Effects of the cations and anions of the additives on charge/discharge performance are discussed. The reduction potential of alkaline cations and radii of halogen anions were correlated with initial capacity and degradation of the metal hydride alloy. A synergistic effect between KOH and some oxyacid salt additives was observed and greatly influenced by the nature of the salt additives. It was suggested that both the formation of a solid film over the metal hydride surface and the promotion of proton transfer in the additives containing electrolytes led to a decreased degradation of the electrodes and an increased discharge capacity. 12 salt additives, NaC2H3O2, KC2H3O2, K2CO3, Rb2CO3, Cs2CO3, K3PO4, Na2WO4, Rb2SO4, Cs2SO4, NaF, KF, and KBr, were found to increase the corrosion resistance of the MgNi-based metal hydride alloy.

Effects of Cs2CO3 Additive in KOH Electrolyte Used in Ni/MH Batteries

“…The changes in surface groups by electrolyte additive consequently change the chemical and physical properties of the alloy particles. In our previous work [52], 32 types of salt additives in KOH electrolytes were tested, and some oxyacid salts were reported to create more surface groups that promoted proton transfer. In the current study, the addition of Cs 2 CO 3 provides C-O and C=O bonds as new active sites for proton transfer.…”

“…In the current study, the addition of Cs 2 CO 3 provides C-O and C=O bonds as new active sites for proton transfer. However, protons bond to the two active sites differently; protons are covalently bound to C-O, but electrostatically bound to C=O [52,66]. With the stronger attraction to C-O, more protons can be bound and later transferred (driven by voltage).…”

“…Surface metal oxidation during charge leads to the formation of surface metal hydroxide, such as Mg(OH) 2 [11,16]. The majority of salt depositions are carbonates and bicarbonates [52]. Weight gains of the alloy electrodes cycled in the Cs 2 CO 3 -containing electrolytes are normalized to that in the 6.77 M KOH electrolyte and presented in Table 3.…”

“…In the test cells, the cathode was sintered β-Ni(OH) 2 , the anode was made from directly dry-compacting the alloy powder onto an expanded Ni substrate without using any binder, and the separator was hydrophilic nonwoven polyolefin. Charge/discharge processes were the same as reported before by Yan et al [52] The cell was charged at 100 mA·g -1 for 5 h, and discharged first at 100 mA·g −1 to a cutoff voltage of 0.9 V. The initial discharge was followed by a 30 s rest for the voltage to recover, and then the cell was discharged at 24 mA·g −1 to reach a cutoff voltage of 0.9 V. The cell was put to rest for 30 s again before the final discharge at 8 mA·g −1 to 0.9 V. Testing for each alloy/electrolyte combination was repeated three times. When the total discharge capacity (sum of capacities at 100, 24, and 8 mA·g −1 ) of the cell decreased by 70%, it was considered to be cell failure.…”

“…Degradation was determined as Yan previously reported [52]. The percent capacity loss per cycle within the initial 10 cycles is shown by the following equation:…”