Rechargeable Li-S Battery with Specific Energy 350 Wh/kg and Specific Power 3000 W/kg. Y. Mikhaylik*, I. Kovalev, J. Xu, R. Schock

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1 / The Electrochemical Society Rechargeable Li-S Battery with Specific Energy 35 Wh/kg and Specific Power 3 W/kg. Y. Mikhaylik*, I. Kovalev, J. Xu, R. Schock Sion Power Corporation 29 E. Elvira Rd., Tucson, AZ, U.S.A., Rate capability improvements of Li-S cells are reported. The main goal of this work, to achieve the maximum high power benefit from the liquid polysulfide cathode system, was realized. Higher rate capability has been achieved through reduction of the electrode stack Area Specific Resistance (ASR) to below 1 Ohm*cm 2, contact design improvements, and better current distribution uniformity along the electrodes and current collectors. Maximal specific power delivered by 2.5 Ah cells at 3 A pulses was 35 W/kg. At a continuous discharge, with specific power at ~15 W/kg, the 2.5 Ah cells delivered specific energy in excess of 2 Wh/kg. Introduction The theoretical specific energy for the Li-S system is 25 Wh/kg compared to 58 Wh/kg for lithium ion and lithium polymer systems. The theoretical energy density for Li-S is 26 Wh/l vs. 18 for lithium ion. The energy potential for Li-S is significantly higher than the best rechargeable system on the market today. However, technological development is still at an early stage. Still, critical problems have already been solved that have allowed Sion Power s lithium sulfur (Li-S) cell design to reach specific energy levels of 35 Wh/kg, surpassing any commercially available rechargeable batteries today. This high specific energy was demonstrated with multiple Unmanned Aerial Vehicles (UAV) flights of a 63 foot wingspan aircraft reaching heights of over 6, ft and remaining airborne for over 54 hours. The UAV cell, and battery pack, designs were optimized for applications with moderate rates and specific power of 1-2 W/kg. The work presented here was focused on improvement of the Li-S cell performance at continuous high current discharge, and delivering high power pulses to allow for a broader range of applications. The study addressed: 1. Component analysis of electrode Area Specific Resistance (ASR), and its improvement. 2. Current distribution uniformity along the electrodes, and its improvement. 3. High power tests of the cell and the impact of electrolyte nature and cell state of charge on high current pulse voltage profiles. 4. Ragone plot analysis for Li-S and other rechargeable systems

2 54 Electrode Area Specific Resistance component analysis and improvement. The Area Specific Resistance of the Anode/Separator/Cathode stack is a function of the electrochemical resistance of the electrodes as well as the electrolyte conductivity in the porous separator and the porous structure of the electrodes. Our study of Li-S cells showed the typical ASR for the porous separator, filled with electrolyte, was around 3 Ohm*cm 2. The ASR of the cathode and anode were in the ranges of 4-1 and 3-1 Ohm*cm 2 respectively. Our study also showed that solvent nature significantly affected Li anode and sulfur cathode electrochemical reaction resistance, changing them by a factor of 3-5 times. However, typical carbon-sulfur cathode electrochemical resistance was always an order of magnitude higher than the anode ASR. Nyqist plots in Figure 1 give examples of such behavior for a Li anode and a carbon-sulfur cathode in two solvents 1,2-Dimethoxyethane (DME) and 1,3-Dioxolane (DOL) Anode cycle #14 Cathode cycle # Charged DOL DME -3 Charged DOL DME Z'' (Ohm) Z (ohms) Z'' (Ohm) Z (ohms) Z' (Ohm) Z (ohms) Z' (Ohm) Z (ohms) Figure 1. Nyqist plots for Li anode and carbon-sulfur cathode in DME and DOL based electrolytes. Electrodes surface area ~ 3 cm 2. The cathode was a major contributor of the whole electrode stack ASR reaching over 1 Ohm*cm 2. Recently Sion Power has found a way of increasing the cathode electrochemical reaction rate by more then an order of magnitude. The ASR of the entire Cathode/Separator/Anode stack has been reduced to 1 Ohm*cm 2. Current distribution uniformity along the electrodes and its improvement. The Sion Power cell is based on a jellyroll design with electrode lengths exceeding 15 cm, utilizing relatively thin conductive cathode substrates (<1 µm) to keep cell weight low and specific energy high. The substrate conductivity, affected by thickness, hindered its ability to provide a desirable rate and uniform current distribution; this was strongly affected by the ASR. Significant ASR reduction required redesigning of the substrate connections.

3 55 Theoretical analysis of the current distribution uniformity along the electrodes typically had been performed through solving the Laplace equation for potential distribution in the electrolyte and considering the electrode polarization due to a limited rate electrochemical kinetics. Laplace s equation is not trivial to solve, even for relatively simple geometries (2). The two parallel strips of Sion Power s cell electrodes with a length L, significantly longer than their width W, allow for simplifying the geometry to one dimensional. The over-potential (η) distribution along the electrode substrates with square specific resistance r S, at steady state conditions, can be described through the Helmholtz equation: 2 d η = 2 dx rs η ASR [1] If the polarization of the electrodes is linear with respect to the current density, or in other words, when the ASR is not a function of overvoltage, equation [1] has a simple analytical solution. The local electrochemical reaction rate distribution, i(x), along the electrodes, with the tab located at one end (x = ), and overvoltage η is: L x Cosh( ) η i( x) = λ [2] ASR L Cosh( ) λ The characteristic length, λ, in equations [2] and [3] is the parameter governing penetration (distribution) of the electrochemical process along the electrodes. It is a function of the substrate resistance and ASR. ASR λ = [3} r S The ratio of λ/l dictates electrochemical rate distribution along the electrode. This dependence is shown in Figure 2 for different values of ASR and one tab connection centered on the electrode substrate. Figure 2 clearly shows that reduction of the ASR from 1 to 1 Ohm*cm 2 required more connection points to the substrate to keep current distribution more uniform. This helped avoid the electrode active material being over-used near the tab connection point. A multiple tab connection analysis showed that the solution for two tabs, one at each end is equivalent to the one tab in the center solution. This follows because segment length is twice shorter than the total electrode length L/2. For N tabs, the optimum design locates tabs, equally spaced along the electrode length, beginning at

4 56 position L/(2N), with subsequent tabs spaced L/N apart. This solution is equal to the one tab solution but considering the segment of electrode length as L/(2N) as shown in Figure 3. The N tab design current distribution uniformity, and the direct current resistance, R DC, for the entire N tab electrode stack are represented by equations [4] and [5]. i(min) 1 = [4] i(max) L Cosh 2Nλ R DC 1 rs ASR = [5] W L 2NTanh 2Nλ 2.5 Rate distribution along the electrode for various Area Specific Resistance 1 tab centered, 158 cm length, 2 mil Li, 12 µ Al foil 1 Ohm*cm2 Rate normilized to Average Current Uniform distribution 1 Ohm*cm Electrode Length (relative) Figure 2. Rate distribution along 158 cm electrode stack at various ASR. N = 4 L/8 Figure 3. Optimal tab connection scheme for 4 tabs.

5 57 By implementing improvements in ASR, cell chemistry and electrode connection design, direct current resistance of the entire cell has been reduced below 2 mohm. High power test and impact of electrolyte nature and cell state of charge on a high current pulse voltage profile. Optimized 2.5 Ah cell designs were able to deliver specific power of 15W/kg at a continuous discharge currents up to 15 A, and over 3 W/kg for 1 s duration high current pulses. It is important to note that pulse power was stable over a wide range of cell Depths of Discharge (DoD). For 1 s pulses the cells generated specific power in the range of 21 W/kg to 25 W/kg at 2 A, and in the range of 32 W/kg to 35 W/kg at 3 A (Figure 4). Specific Power, W/kg A 2 A % 2% 4% 6% 8% 1% Cell DoD Figure 4. Specific Power vs. Cell DoD for 2 and 3 A pulses. Pulse duration 1 s. The high current discharge profile was a function of the electrolyte nature and, particularly, the ability of the electrolyte to dissolve high amounts of polysulfides. At the beginning of the cell discharge, when solid elemental sulfur is transformed into soluble polysulfides, the high polysulfide soluble electrolytes lead to a voltage increase under high current pulses while the low soluble electrolytes lead to voltage reduction (Figure 5).

6 A x 1 s Discharge Pulse Profile Specific Power ~1.3 KW/kg S 8 + 2e - + 2Li + Li 2 S 8 Cell Voltage 2.1 High Li 2 S 8 Solubility Electrolyte 2.5 Lower Li 2 S 8 Solubility Electrolyte Pulse Duration, s Figure 5. 1 A pulse voltage profile for cells with high and low polysulfide soluble electrolytes. The high current pulse profile was also a function of the cell State of Charge (SOC), you may think of SOC as the degree of sulfur conversion. When all polysulfides were in the liquid state the voltage was very stable even at 2 A pulses (Curve 2, Figure 6). Generation of lower solubility polysulfides, particularly Li 2 S, at low SOC, leads to voltage decrease at the end of pulse (Curve 3, Figure 6) A x1s (~2.5KW/kg) Pulse Disharge Profiles at Various Stages of Sulfur Conversion 2 Li 2 S 8 +2e - +2Li + 2Li 2 S 4 Cell Voltage S 8 +2e - +2Li + Li 2 S 8 Solid Li 2 S 4 +6e - +6Li + 4Li 2 S Solid Pulse Duration Figure 6. 2 A pulse voltage profile for cells with various SOC.

7 59 Ragone plot analysis for Li-S and other rechargeable systems Specific energies delivered by Li-S cells at a wide range of discharge currents (specific powers) allowed us to generate a Ragone plot (Figure 7) and compare Sion Power Li-S cell with other commercially available rechargeable systems. Specific Energy, Wh/kg Li-ion 1865 Ni-MH Ni-Cd Sion Power Li-S Li-ion A Specific Power, W/kg Figure 7. Ragone Plots of Rechargeable Electrochemical Systems Ragone plot comparison showed that the Sion Power Li-S system over-performs all other rechargeable systems in terms of delivered power and energy. The substantial rate capability improvements, obtained through the above described modifications to Sion s previous Li-S cell design, opens up a wider range of applications for Li-S cells. Unlike high power Li-ion cells which deliver specific energy in the range of 64 W/kg to 18 Wh/kg (3, 4), Sion Power s Li-S high powered system offers specific energy exceeding 3 Wh/kg. References John Newman and Karen E. Thomas-Alyea,-3rd ed. Electrochemical Systems, p. 419, Wiley Interscience, NJ (24) _27.pdf 4.