The most widely used battery technology for multi-hour grid energy storage is NGK Insulator's high temperature (300 to 350C) sodium sulfur (NAS) battery. The high operating temperature is required to obtain adequate mobility for the sodium ions through the solid ceramic beta-alumina electrolyte. The largest installation of such batteries is 204MWhr installation supporting a wind farm in Rokkasho Japan. This installation opened operations in 2008. The sodium sulfur chemistry is attractive from a scalability point of view since both sodium and sulfur are earth abundant elements.
High temperature operation creates safety concerns, and, in fact, a battery fire occurred on September 21, 2011 at one of NGK installation's in Japan, leading to a shutting down of all NAS battery storage installations world wide for a long period of time. In June of 2012 NGK Insulator's published a report describing the causes of the fire and safety enhancements which they felt provided sufficient operating margin to justify reopening their battery factory. This presentation mentions two California installations (One in San Jose and one in Vacacville) of NGK's NAS batteries which have occurred since the safety enhancement were announced. This technology is not dead, but my sense is that enthusiasm for it has waned in spite of the enhanced safety features. These batteries are too expensive ($320/KWh) to truly revolutionize grid operations but they have found some niche markets.
From time to time I stumble upon research work exploring options to lower the operating temperature of NAS batteries. If these ideas introduce rare elements to the battery chemistry they may not have the same long term scalability as the current chemistry. For example this abstract claims that adding cesium to the liquid sodium cathode will allow the operating temperature of sodium sulfur batteries to be lowered to 150C. Unfortunately cesium is a rare element which cost about $40,000/Kg. The effect of the cesium is to enhance to wettability of the sodium on the solid beta-alumina electrolyte. Conceivably some other lower cost metal can be found which produces the same effect, but such an option awaits future research.
Recently an interesting low temperature NAS battery design has emerged from scientists (Dr. Gao Liu, Dr. Dongdong Wang, and Dr. Kehua Dai) at the Lawrence Berkeley Laboratory. They claim that this battery can operate at a temperature of 80C. This temperature is below the melting point of pure sodium (97.7C). A liquid metal anode avoids the problem of dendrite formation by solid sodium and is a key enabler of the long cycle life (4500 cycles) of current NAS batteries. Liu et al maintain this feature of the NAS battery by using an alloy of sodium and potassium which has low melting temperature. The solid beta alumina electrolyte is replaced by a solid polymer which conducts sulfur ions (rather than sodium ions) at low temperature. Their battery design also includes a new cathode which is a mixture of sulfur and a conductive polymer. Some details of the cathode and electrolyte chemistry and structure can be found in this patent
As this application for Cleantech to Market funding shows, Liu et al seem to believe that this battery architecture is not just a laboratory curiosity but has the potential to be manufacturable. I believe that the manufacture of the tubular solid ceramic electrolyte for the current high temperature version of the NAS battery is an expensive process, so that the replacement of this component by a polymer electrolyte potentially represents a significant cost savings. Some information about projected development time lines manufacturing costs for of these batteries is given in this presentation. The numbers in the presentation are probably highly speculative. Furthermore the LBL scientists themselves will probably not follow this technology to maturity but will attempt to license it to a private company. I would not hold my breath waiting for these batteries to start rolling off the assembly line in large quantities, but I am always interested in technologies which have potential long term scalability.
April 3, 2015