Li-S Batteries

The electric car is considered as the most promising technical solution for automotive transportation since the use of electric energy will not only slow down the consumption of petroleum resources but also contribute to the reduction of emission of carbon dioxide and toxic air pollutants. With stricter environmental regulations, for example, due to the “Corporate Average Fuel Economy (CAFE)” regulation in Europe, the CO2 emission of cars has to be lower than 130 g/km. Automobile manufacturers are now urged to produce electric or hybrid vehicles as promoted by the EU “Directive on Deployment of alternative fuels Infrastructure” allowing the increase of xEV and consequently the associated battery market. Other electrochemical or chemical solutions could be proposed in response to the objectives of the NMP17 call. It is clearly illustrated in the Ragone plot in Figure 1 [1,2,3] that these will not achieve the target of 400Wh/kg. Indeed current lithium ion technology has a specific gravimetric energy of 210 Wh/kg, also flow batteries exhibit both lower energy and power densities with a typical gravimetric energy density between 40 to 70 Wh/kg. Therefore, for on-board technologies, redox flow batteries will not reach the value of 400 Wh/kg as required in this specific NMP17 call. Modification on thermal and electrical management or architecture of redox flow systems will inevitably lead to increased weight and volume, with little chance of doubling the specific energy required. Manufacturing and material cost with high internal resistivity leading to low power are the majors drawbacks for the all solid batteries. Sodium based technologies are limited but remain a critical candidate regarding its electrochemical possibilities. Unfortunately, sodiumbased technology presents major constraints regarding handling and manufacture due to its high reactivity with air and moisture. Drastic change at the industrial level must be done in order to enable work with sodium. Metal air technologies (e.g. Li, Na, Mg, Al) are potentially interesting in term of higher specific energy theoretically reachable. The two major drawbacks for the use of metal air technologies are the difficulties linked to the scale-up of a working device and their lack of power density.

Current metal air technology could not be charged at sufficiently high C-rates without a significant capacity drop – thus it would have the further complication of then having to be integrated with other technologies with high specific power, such as supercapacitors. It should be noted that even though the industry uses the nomenclature metal air, the actual technology is metal oxygen. Other components of air, for example carbon monoxide, poison the expensive catalysts used in this technology.

Also metal oxygen technologies are currently at a TRL level below 4, which is required as a starting point for this call.

Li-S_figure_1

Figure 1: Ragone plot adapted from [1,2,3], CAPA for electrical capacitor, SMES for Superconducting Magnetic Energy Storage, Metal air for Lithium, Sodium, Magnesium and Aluminum air technologies, Flow batteries for Zincbromine, Polysul de-bromide, Vandium Redox Batteries.

There is no one single perfect technology for all applications, but one technology adapted to one specific use. Lithium sulphur technology has been selected for ALISE project from the range of mechanical, chemical, electrical and electrochemical technologies available. This emerging technology presents similar abilities of charge / discharge C-rates in respect with onboard technology, with a potential to double specific energy. Figure 2 represents the actual commercial roadmap of OXIS energy where specific energies of 400 and 500 Wh/kg are planned for 2016 and 2018 respectively. However the lithium sulphur electrochemistry is still not fully understood and not optimized yet.

The cell operating voltage is much lower in respect to lithium ion and therefore modifications are required regarding associated cell, module and battery packs, and will be addressed in the ALISE project.

Li-S_figure_2

Figure 2: Projection of OXIS LiS technology.
Sources http://www.oxisenergy.com/tech

The inherent mechanistic pathways of Lithium-sulphur technology are different to those of lithium ion. The electrochemistry of conventional lithium ion storage systems follows the reaction of insertion and de-insertion of lithium cations at the graphite anode or lithium based cathode. This is not the case for the lithium-sulphur electrochemistry where a succession of solubility processes take place. The polysulfide (PS) shuttle is assumed as a stepwise electrochemical reduction of sulphur with polysulfides of different chain lengths as intermediates, which mainly include Li2S8, Li2S6, Li2S4, and Li2S2. [4]. Due to the insulating nature of sulphur and its successive by-products, the reduction of sulphur and PS can only take place on the surfaces of conductive carbon. The soluble PS dissolves into the electrolyte solution, which leaves the remaining sulphur exposed to the conductive carbon so that the reduction can progressively move forward. The discharge process can be divided into four reduction regions [5] as illustrated in Figure 3:

Li-S_figure_3

Region I: At the cathode, from solid to liquid phase (voltage plateau 2.2 – 2.3): S8 + 2Li --> Li2S8.

Region II: Liquid – liquid phase, with an increase of the viscosity: Li2S8 + 2Li --> Li2S8-n + Li2Sn.

Region III: Formation of insoluble discharge by-product, from liquid to solid, major contribution of the LiS cell capacity (voltage plateau 1.9 – 2.1 V): 2Li2Sn + (2n-4)Li --> nLi2S2 and Li2Sn + (2n-2)Li --> nLi2S.

Region IV: solid-solid reduction from insoluble Li2S2 to Li2S: Li2S2 + 2Li --> 2 Li2S.

The redox shuttle phenomenon occurs due to the diffusion of the soluble high order polysulphides (Li2S8, Li2S6) through the separator and onto the anode surface where they reduce to lower order and partially soluble lithium di and mono sulphide species, therefore some of the active sulphur is lost with each cycle. Furthermore due to the diffusion of the high order polysulphides the oxidation potential of elemental sulphur is not obtained on charging and therefore the full capacity of the cell is not achieved. This is a major degradation mechanism within the LiS technology and the ALISE project will address this issue through investigations into the anode, cathode and electrolyte materials as detailed below. During region I and II in Figure 3, the cell suffers from the highest self-discharge rate and the cell’s theoretical capacity can be seldom obtained. Beside the electrochemical reductions, chemical reactions between polysulfide anions are also present in the electrolyte solution, which are affected by the type of solvents, concentration and temperature of the polysulfide solution: Li2Sn + Li2S --> Li2Sn-m + Li2S1+m and Li2Sn --> Li2Sn-1 + 1/8 S8.

Notes :

1 H. Chen, T. N. Cong, W. Yang, C. Tan, Y. Li, and Y. Ding Progress in Natural Science 19 (2009) 291–312.
2 Shyamal K. Das, Sampson Laub and Lynden A. Archer bJ. Mater. Chem. A, 2014, 2, 12623–12629.
3 B.G. Pollet et al. Electrochimica Acta 84 (2012) 235– 249.
4 R. D. Rauh, F. S. Shuker, J. M. Marston and S. B. Brummer, J. Inorg. Nucl. Chem., 1977, 39, 1761–1766.
5 S.S. Zhang J. Power Sources 231 (2013) 153 – 162.