I have written previously about NGK Insulator's large format molten salt Sodium Sulfur Batteries which they are selling in the utility scale storage market. Trans Ionics Corporation is developing a new form of Sodium Sulfur (NaS) battery which they claim will have performance characteristics which will allow it to compete in the EV and PHEV markets. Some details about the design of these battery taken from their web site are given below:
Trans Ionics Corporation through its subsidiary, Natrion Energy Corporation, is developing an Advanced Thin Film Sodium Sulfur (NaS) Battery Module that incorporates innovative technologies offering the following benefits in EV and PHEV applications:
A conventional NaS battery has three components: (1) an anode compartment, (2) a solid electrolyte and (3) a cathode compartment. Sodium Beta Alumina (Β”-Al2O3) is the electrolyte of choice. This solid electrolyte is permeable only to sodium ions (Na+) and prevents the other cell components (sodium and sulfur-rich sodium polysulfides) from coming into physical contact.
Sodium metal melts at 98 °C and sodium polysulfides have even higher melting points; so in order to provide the ionic mobility necessary for high power density, the system must be maintained as a molten salt, which requires a battery operating temperature in the range 300 – 350 °C. As a precaution against mechanical breakage, electrolytes in the past have been fairly thick (averaging 1-2 mm in thickness). So, the two major problems with sodium-sulfur batteries to date have been (1) the requirement to operate at high temperatures, which is, in part, caused by the thickness of the electrolyte and in part caused by the high viscosity of a molten salt system, and (2) their weight.
The NaS battery under development by Trans Ionics overcomes these problems by (1) significantly reducing the thickness of the electrolyte and (2) operating with a proprietary mixture of components in the anode and cathode sections both of which permit operation at far lower temperatures than previously thought possible for NaS batteries.
As can be seen, reducing the thickness of the electrolyte layer to 5 μm (from 1,000 μm) results in a drop in ASR (My Note: ASR=Area Specific Resistance) of a factor of 200 over the conventional NaS battery. With the other improvements proposed, this allows operation at 25 °C with a voltage drop of only 0.09 V. Even if the conventional battery could be operated at 25 °C, the resulting 18.6 V loss would render it inoperable. So the first advancement is an ultrathin solid electrolyte.
The second advancement is the development of a mixture of components that facilitate operation of a NaS battery at lower temperatures. While they are proprietary, it can be said that they involve the use of liquid species that enhance the solubility and conductivity of both the sodium polysulfide cathode and the sodium anode thus eliminating the need to run as molten salts at elevated temperatures.
The Ford and NGK processes use electrophoretic deposition of Β”-Al2O3 onto stainless mandrels to form the electrolytes. Because these were stand-alone electrolyte tubes, they had to be thicker to protect against catastrophic rupture. The proposed process uses tape casting to produce a microporous substrate and screen printing or other techniques (depending on the desired thickness) to apply the dense electrolyte. Because these batteries will be operated at 25-100 °C, conventional, flexible epoxy seals are planned and materials of construction can and will include plastic as well as thin metallic components that are both lightweight and cost effective.
Rechargeable batteries can have a wide range of energy densities as shown in the figure. Of the batteries under serious consideration for electric vehicles, lithium ion and lithium polymer batteries are the most advanced but are still limited to system level Specific Energies of <160 Wh/kg.
The highest Specific Energy and Energy Density is demonstrated by alkali metal sulfur batteries: lithium-sulfur (LiS) and sodium-sulfur (NaS); and these will be the focus of the balance of this review.
Sodium has a higher molecular weight than Li and the formation of Na2S has a lower free energy of formation (~ -350 kJ/mol). Nevertheless, the battery still has a theoretical Specific Energy of ~ 1,250 Wh/kg or still twice that of the Li ion battery.
There are two major issues with LiS cells as they currently exist: (1) development of rough lithium surface morphology and (2) Li/liquid electrolyte depletion. In the first case, dissolution and redeposit of Li over a number of charging and discharging cycles creates (a) dendrites extending from the surface which can pass though the porous polymeric separator and short out the cell or (b) porous “mossy” lithium deposits which can absorb electrolyte leading to premature anode disintegration. In the second case, loss of solvent due to reaction with Li prevents proper operation of the cathode; and the reaction products increase cell impedance. The proposed advanced thin film NaS battery contains a solid (not a liquid) electrolyte that does not exhibit either of these problems.
As stated previously, the theoretical Specific Energy of the NaS reaction is 1,250 Wh/kg. Unit cells from commercial NaS batteries sold by NGK exhibit 367 Wh/L and 222 Wh/kg which are limited by electrolyte thickness and cell resistance. Unfortunately, these values are reduced to 160 Wh/L and 118 Wh/kg (only 9.4 % of theoretical capacity) when the system is assembled due to the difficulty of running at 350 °C.
The proposed thin film NaS battery will decrease the thickness of the electrolyte by a factor of 200 and allow the system to operate efficiently at 25 °C. The unit cell is expected to exhibit at least 600 Wh/L and 475 Wh/kg and the full cell is expected to show at least 300 Wh/L and 300 Wh/kg (for 24% of theoretical capacity).
Key technical risks are (1) producing a 5 μm thick electrolyte that is pinhole-free, (2) determining the optimum composition for the anode and cathode fluids to minimize resistance and (3) utilizing the lightest components possible for the cell to minimize the weight. The technology is currently at TRL-3 for the electrolyte and TRL-2 for the electrodes; however, we believe that with appropriate funding, the technology can achieve TRL-6 within 24 months. (My Note: TRL stands for Technology Readiness Level. TRL-6 is prototype demonstration)
Transionics Corporation, the parent company of Natrion Corporation, published a paper in 2004 concerning the use of a thin film beta alumina (Al2O3) solid electrolyte in an NaS battery. At that time they were proposing to use the battery in reverse as an electrolytic cell to separate sodium from sulfur. They were planning to use this electrolytic cell as part of a process called Transfining-R which was to be used for removing sulfur from sour crude.
Now, in addition to the thin film electrolyte they are claiming to have developed an alteration to the chemistry of the liquid electrodes which will allow low temperature operation and much higher energy and power densities than NGK Insulator's design. If the advertised energy and power densities are realized in a manufacturable design then this battery chemistry could compete with lithium ion batteries in mobile applications.
However, I would not hold my breath waiting for these batteries to start rolling off the manufacturing lines in high quantities. TRL-2 and TRL-3 are early in the development process, and Natrion's claim that prototype demonstration (TRL-6) could possibly be achieved within 24 months must be regarded as highly conjectural.
October 21, 2010Energy Storage News
rogerkb [at] energystoragenews [dot] com