In situ electrochemical investigations of the kinetic and thermodynamic properties of nickel-metal hydride traction batteries

Although large ampere hour nickel-metal hydride (Ni-MH) traction batteries, are in the stage of being commercialized for electric and hybrid vehicle a pplications, little is known about their performance characteristics. By us ing a standard Hg/HgO reference electrode in a commercial Ni-MH battery we were able to conduct in situ measurements to determine both kinetic and the rmodynamic properties of the system, including the characteristics of indiv idual electrodes. Using the galvanostatic intermittent titration technique (GITT), we simultaneously and effectively determined the open-circuit volta ge of the battery, the equilibrium electrode potentials, and the diffusion coefficient of proton and hydrogen in the nickel and metal hydride electrod e, respectively, as a function of the states of charge (SOC). Using the cur rent-step excitation technique, we found that the internal resistance of th e battery primarily comes from the metal hydride electrode, which is greate r by one order of magnitude than that of the Ni electrode. The cyclic linea r micro-polarization experiments, on the other hand, showed that the charge -transfer resistance of the electrochemical reaction at the metal hydride e lectrode is about twice larger than that of the Ni counterpart above 20% SO C. In comparison, the internal resistance is an order of magnitude smaller than those of the electrochemical charge-transfer reactions. The micro-pola rization technique also allowed us to calculate the exchange current densit ies of the respective electrode electrochemical reactions and the associate d specific exchange current densities. These in situ, simple but detailed, characterizations of the thermodynamic and kinetic properties of the Ni-MH system provided valuable information for better understanding of the batter y performance. (C) 2001 Elsevier Science BN. All fights, reserved.