Metal hydrides for high-power batteries
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Introduction In spite of the present interest in developing and commercializing lithium (Li) battery powered electric vehicles for the mass market, 2012 was a year of unprecedented collapse in the Li battery sector, as some manufacturers went bankrupt, dissolved joint ventures, wrote off assets, or restructured operations in an attempt to survive. In the United States, A123, Valance, and Ener1 filed for Chapter 11 protection. All of the principal players in the industry are steering away from battery-powered electric cars because they cannot make money selling batteries at prices carmakers can pay.1 Interestingly, the first battery-powered electric car was invented by Thomas Davenport in mid-1830s.2 By the end of the century, electric vehicles were well established and more frequent in the streets than cars with internal combustion engines.3 The electric car started easily and accelerated smoothly because electric motors have good torque even at a low speed. They were silent with no emissions, and no oil company could be accused of stopping alternative energy solutions. Despite all benefits, and a half century head start, the electric car was quickly abandoned when Otto and Diesel’s engines came, because battery development was too slow. Rechargeable batteries are not as rechargeable as we often would like to believe. When a battery is charged or discharged
with nonzero currents, various temperature, concentration, voltage, and current gradients are created. These gradients are created over complex interfaces in the battery cell, between electrodes and electrolytes, between conductors and electrode materials, and between the charged and discharged parts of the electrode. These gradients drive a variety of unwanted and complicated interconnected parasitic side reactions. If the electrode reactions involve dissolution/precipitation of different phases, where atoms and molecules are moved around, they are more likely to derail the battery cell upon cycling. In older battery chemistries, some of these side reactions have been given trivial names such as sulfation or memory effect.4 All reactions, including the side reactions, have to be exactly reversed when shifting from discharge to charge. The inability to fully reverse all the reactions will make rechargeable batteries unstable with respect to charging and discharging. To minimize this instability, it is important for the electrodes to work under as ideal conditions as possible. Gradients and their effects must be minimized. This puts a high demand on the production quality, with an emphasis on uniformity of electrodes and separators with respect to porosity and purity. Battery development is thus about minimizing side reactions by optimizing the design of the battery components or by
Zhou Ye, Höganäs AB, Sweden; [email protected] Dag Noréus, Department of Materials and Environmental Chemistry, Stockholm University, Sweden; [email protected] John Richard Howlett III, Nilar Inc., Colorado, USA; [email protected] DOI: 10.1557/mrs.2013.109
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MRS BULLETIN • VOL
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