Vacuum and Atmospheric Coating and Lamination Techniques

Applied to Li-S Battery Fabrication

John Affinito, Yuriy Mikhaylik, Chariclea Scordilis-Kelley,

Chris Campbell, Tracy Kelley, Amy Ng and Kyle Smith

All of Sion Power Corporation, 2900 E Elvira Road, Tucson, Arizona 85718

Abstract: Lithium-sulfur batteries offer the highest theoretical specific energy of any

electrochemical couple involving only solid elements, more than four times that of the highest

energy lithium-ion chemistries in use today. To date, commercialization has been slow due to

cycle-life limitations. For commercialization Li-S batteries must deliver their high energy safely,

and for many hundreds of discharge/charge cycles. The major Li-S cycle life failure

mechanisms are traceable to solvent reactions with the metallic lithium anode, while safety is

dominated by anode reactions with sulfur. However, specific and volumetric energy are

controlled by cathode structure, solvent uptake and cell design efficiency. In this paper the

fundamental aspects of lithium-sulfur chemistry are reviewed, the primary lithium-sulfur failure

mechanisms are discussed and Sion Power’s approaches to increase cycle life, increase specific

and volumetric energies and improve safety are presented. These approaches include: doped,

vacuum deposited lithium, utilizing a sputtered current collector on a thin film releasable

substrate; a combination of vacuum deposited lithium ion conducting ceramic/polymer multi-

layer coatings; an atmospherically coated polymeric separator layer; atmospherically coated

cathode primer and cathode layers with engineered porosity; and uniaxial pressure applied to the

packaged cell. Cycle life performance data is presented for the first cells produced incorporating

all of these new technologies.

Introduction: Sion Power, and Sion’s joint development partner BASF, are developing lithium

sulfur (Li-S) batteries to be practical, economic and safe for powering electric vehicles (EVs).

The theoretical specific energy and energy density of Li-S are (2,550 Wh/kg, 2,860 Wh/l),

compared to about (600 Wh/kg, 1,800 Wh/l) for Li-ion. The highest energy commercial Li-ion

cells are approaching their practical limit, of about 40% of theoretical cell level specific energy,

placing them below about 240 Wh/kg in the packaged cell. At only 22% of theoretical cell level

specific energy (560 Wh/kg), fully packaged Li-S battery packs would power a 1,500 kg, 5

passenger family vehicle using full utilities, more than 300 miles between charges, with battery

pack weight less than 300 kg. [1] Sion already has a commercial 350 Wh/kg battery (at only

14% of theoretical cell level specific energy) that is sold into the Unmanned Aerial Vehicle

(UAV) market. Commercial viability for automotive electric vehicles (EVs) requires safe cells,

low cost and between 500 and 1,000 full depth of discharge cycles. In 2011 dollars, the

estimated pack cost with the materials and processes to be discussed below, once high volume

EV battery manufacturing is underway (in 2020), is less than $250/kWh.

For conventional Li-S batteries, with specific energy exceeding 350 Wh/kg, where none of the

advanced technologies to be discussed later are used, Sion has found the following to be true:

Energy - Specific Energy (Wh/kg) is limited by two primary mechanisms:

– By what is called the Polysulfide Shuttle, where soluble polysulfide species diffuse from

the cathode and chemically self-discharge on the anode;

The shuttle can be suppressed by Sion’s patented NOx additives, or by physical

membranes protecting the anode; and

– By precipitation of the end of discharge product (Li2S) within the cathode porosity. The

Li2S clogs the cathode porosity and shuts down cathode function, ending the cycle before

all of the energy is extracted from the sulfur.

Li2S precipitation can be mitigated by either, or both, of:

o Solvents more aggressive to Li2S in the cathode, or higher levels of solvent; or

o A cathode topology/microstructure engineered to be less susceptible to clogging.

Cycle Life - Solvent depletion, due to the metallic Li anode reacting with electrolyte solvent,

limits cycle life.

– The observable failure, though, is on the cathode side, because the decrease in solvent

level (due to the Li-solvent reaction) starves the cathode of solvent and increases Li2S

precipitation. Cathode function then decreases with each cycle until the cell is considered

dead. But this is an anode problem.

NOx additives, that inhibit the polysulfide shuttle, do not stop solvent depletion due to

Li-solvent reactions, though physical membranes can.

Safety - Safety is limited by metallic Li reacting with sulfur species.

– The mechanism is thermal runaway when the temperature exceeds about 130 °C, and

sulfur diffusion to the anode, and rapid reaction with the Li, can not be stopped.

With Sion’s NOx shuttle inhibiting additives, sulfur diffusion due to the polysulfide

shuttle is not an issue under normal operating conditions, but the inhibition decreases

with increasing temperature, and is not functioning at 130 °C

o Li-S cells fail benignly under all other abuse scenarios – except thermal runaway.

Many groups have attempted to improve Li-S cycle life and specific energy through chemistry.

[2, 3, 4] To balance rate with cycle life, many solvent formulations, salts and additives were

reported in the literature. [3, 5, 6, 7] Sion research shows Li-S cycle life and safety are

dominated by the anode. Metallic Li reacting with electrolyte solvent is the primary cycle life

issue, while metallic Li reacting with sulfur is the main safety issue. Sion is addressing these

anode problems with unique new physical barrier technologies. [8] However, specific energy

and energy density are dominated by cathode structure, solvent uptake and cell design efficiency

issues. [9, 10] The cathode can become clogged with discharge products and the electrolyte

solvents may not easily resolubilize the reaction products. Sion is addressing the cathode issues

through engineered topology, porosity, and microstructure, as well through better solvents.

Sion’s current commercial UAV cell is a wound prismatic format with specifications (5.8 Wh,

2.1 V, 2.8 Ah, 350 Wh/kg, 320 Wh/l, ~30 cycles). With the innovations to be discussed later,

Sion is targeting a stacked plate format EV cell in 2014 with specifications of (50 Wh, 2.1 V, 25

Ah, 600 Wh/kg, 600 Wh/l, 500 cycles), with cycle life increasing to 1,000 by 2016. Such

performance, when attained, will easily meet the commercial EV requirements discussed above.

To attain this performance, the materials, processes and design that Sion and BASF are pursuing

are summarized in Figure 1.

Figure 1. Roadmap to Sion/BASF’s Future Li-S EV Cell.

Six anode protection strategies (labeled - in Figure 1

are being implemented in the following nine steps (a-i).

a. A smooth releasable substrate is atmospherically

coated onto a carrier substrate to improve Li morphology

and reduce weight. b. An anode current collector is

vacuum coated onto the releaseable substrate, improving

morphology during cycling and allowing cycling at 100%

Li depth of discharge. c. Vacuum deposited Li (VDLi) is

coated over the current collector, which is smoother than

Li-foil and can be doped during deposition. d. The VDLi

is vacuum over-coated with a ceramic-polymer multi-

layer anode stabilization laminate (ASL) to block solvent

and polysulfide migration. e. To provide a compliant

pad, a physical barrier and the reservoir for the anode

component of a dual phase solvent system, a polymer separator layer is then atmospherically

coated onto the Carrier/Release/CurrentCollector/VDLi/ASL. f. The entire anode assembly is

slit to width. g. The carrier substrate is stripped away, increasing specific and volumetric energy

as only the releasable substrate is incorporated into the cell. The carrier is only required to

withstand the deposition heat load and handling stresses during processing. h. Two rolls of

released anode assembly are laminated back-to-back, producing a finished anode that is a

substitute for Li foil, but with a current collector and multiple levels of physical barrier layers

embedded within. i. Protected anode material is then brought together with cathode material, in

a stacked plate prismatic format, and packaged under externally applied pressure. Pressure

eliminates mossy Li and stabilizes the ASL layers. Each of these steps provides engineering,

materials, chemistry and processing challenges. The engineered porosity cathode and cathode

primer are atmospherically coated.

Li-S Basics: The left hand side (LHS) of Figure 2 is the theoretical discharge curve highlighting

Li-S’s signature double plateau discharge. On the high voltage plateau (HVP) S8 reduction to

Li2S4 dominates the electrochemistry as the cell discharges. All HVP species are very soluble.

On the low voltage plateau (LVP) the reduction of Li2S4 to Li2S is completed. The LVP species

have low solubility and Li2S precipitates as a solid. After generation of soluble polysulfides on

the first discharge, the soluble polysulfides freely diffuse from cathode to anode. Because of this

diffusion driven polysulfide shuttle, conventional Li-S cells fail to recover about 1/4th

of their

theoretical sulfur capacity after the first discharge, so sulfur utilization never exceeds ~75%. [11]

The effects of the massive polysulfide shuttle induced self-discharge (losing 1/4th

the capacity)

are seen in the 900 mAh/gS discharge curve (red curve) on the right hand side (RHS) of Figure

2, where both the HVP and LVP are truncated. The missing UVP capacity is due to the shuttle

because the soluble species diffuse to the anode and are reduced more quickly than the cell can

be charged. Sion has developed electrolyte additives, based on nitrates like LiNO3, that inhibit

the shuttle. Physical barriers (membranes) can also inhibit the shuttle. With the shuttle

inhibited, sulfur utilizations, in conventional Li-S cells, of about 72% are seen for discharge rates

between about C/3 (3 hour discharge) and C/5, as seen in the 1,200 mAh/gS discharge curve

(blue curve) in the RHS of Figure 2. At C/24, or slower discharge rates, sulfur utilization can

approach 100% when both the missing LVP capacity is recovered and the shuttle is inhibited.

Figure 2. Left Side: The theoretical Li-S discharge curve can be approximated quite closely with real cell designs, but only at very low discharge rates (days). Right Side: The theoretical Li-S discharge curve (green curve, 1,672 mAh/gS) compared with real discharge curves with: strong shuttle and old, clogging susceptible, cathode technology (red curve, ~900 mAh/gS); with the shuttle controlled with NOx electrolyte additives, but old clogging susceptible cathode technology (blue curve, 1,200 mAh/gS); and with shuttle controlled with new anode protection technologies and new clogging resistant cathode structures (brown curve, 1,450 mAh/gS).

The difference in LVP sulfur utilizations, between C/3 and C/24, is not related to the shuttle, or

to nitrate, or to physical barrier membranes. The decreased LVP sulfur utilization is due to Li2S

precipitation and plugging of cathode porosity. As porosity blocks, cell polarization increases

rapidly, cell voltage drops and the remaining ~25% of LVP capacity cannot be recovered at C/3.

That all the non-recovered capacity is on the LVP is indicative of Li2S precipitation/porosity

blocking. The HVP retains its theoretical capacity. At C/24, or slower, the reaction has time to

equilibrate across the cathode and most LVP capacity can be recovered. By engineering cathode

topology, porosity and microstructure more efficiently, Sion has achieved >85% S-utilization at

C/5, as in the 1,450 mAh/gS discharge curve (brown curve) in the RHS of Figure 2.

Sion has a complete, first principles, electrochemical model for the Li-S cell, which includes

precipitation and cathode porosity blocking. [12] The plot in Figure 3 shows the model

predictions for cathode porosity at the end of both the UVP and LVP for conventional Li-S,

clogging susceptible, cathodes. The model indicates the cathode porosity remains completely

open at the end of the HVP, but rapidly blocks and is nearly completely blocked, on the anode

facing surface, by the end of the LVP. In fact, the model indicates the polarization, due to this

blocking, is what leads to the end of a C/5 discharge at <72% sulfur utilization. The model

predicts that, at ~72% sulfur utilization, the baseline cathode surface facing the anode has open

volume about equal to the percolation limit of this type of cathode structure (~16%). The

resulting cell polarization, due to this blockage, terminates the LVP and the cycle is ended.

Figure 3. Porosity is not blocked on the Upper Plateau (blue trace) of conventional Li-S cathodes due to the high solubility of upper plateau polysulfides. For conventional Li-S cathodes, blocking begins as the lower plateau discharge begins, due to the low solubility of the Li2S species. Polarization spikes up, ending the cycle as blockage of the anode facing surface of the cathode reaches the percolation limit (~16% open, red trace) at a sulfur utilization of ~72%.

SEM photos in Figure 4 show, for a commercial cell, with a conventional clogging susceptible

cathode, how the pore blocking manifests. However, Sion has compared pristine and fully

discharged cathodes, with engineered topology, porosity and microstructure, and found the

cathodes to not clog at any state of discharge (not shown). The new cathode technology results

in higher sulfur utilization (>85%), and in modest cycle life increases. The improved capacity of

the cells with the clogging resistant cathodes is seen in Figure 5. As seen in Figure 5, capacity

decreases with each cycle with conventional cathodes, while capacity initially increases then

remains high much longer when the clogging resistant cathode technology is employed.

As seen in Figure 2, nitrate additives, or physical barrier membranes, will interrupt the shuttle

and recover 1/4th

of the theoretical sulfur capacity. However, nitrate does not protect the metallic

lithium anode from reacting with solvent and thin membranes are fragile and have limited

lifetimes when used alone. Using thin ceramic/polymer membranes, when combined with a

compliant gel pad and pressure can stabilize the thin membranes and improve both safety and

cycle life. Figure 6 shows the improvement in cycle life obtained with two different

combinations of the anode protection and advanced cathodes discussed above, compared to the

conventional Li-S cell technology.

Figure 4. Conventional Li-S cathodes are susceptible to clogging, particularly on the anode facing surfaces shown here. Sion has engineered a cathode topology, microstructure and porosity that is much less susceptible to this problem, as seen in the cycling performance of Figure 5. Following the blue arrows leads through successively higher magnifications of the anode facing cathode surface at the end of the lower voltage plateau where, in the lower left corner, precipitated Li2S nodules are clearly seen. This clogging is the cause of the cycle ending at <75% S-utilization, instead of 100%, and as predicted by the model in Figure 3.

Figure 5. Left: With conventional cathode structures, porosity begins to plug immediately causing noticeable capacity loss with each cycle after the first discharge, due to polarization induced by the Li2S precipitate blocking the cathode porosity. Right: With an enhanced, engineered porosity, cathode that resists blocking, the capacity actually increases for the initial cycles (as the sulfur becomes evenly distributed), and the capacity remains high much longer – even without anode protection inhibiting solvent loss.

Figure 6. The first cells using the new anode protection technology with engineered porosity cathodes are already showing a 3x increase in cycle life (at ~350 Wh/kg), and the cells are still cycling.

Figure 7. Compliant polymer gel also improves cell safety, as in this thermal ramp data. When combined with cycling under pressure, the gel improves safety due to: a reduced sulfur reaction rate with Li due to the smoothness of the Li under pressure; and to the controlled reaction of the special polymer with the Li. Without pressure the Li surface becomes mossy, with high surface area, and higher surface area Li reacts more quickly than smooth Li.

Differences between the surface smoothness/area of Li cycled with and without pressure are

vividly seen in Figure 8. With pressure producing reduced surface area/reaction rate, under

thermal runaway conditions the Li can react slowly with the special gel polymer, as opposed to

quickly with the sulfur.

Figure 8. By inhibiting the evolution of high surface area “mossy” lithium, applied pressure reduces the lithium solvent reaction rate, aiding in increasing cycle life, and makes the cell safer by inhibiting thermal runaway reactions of lithium with sulfur.

With the ASL coated over the VDLi, if pressure is applied to the cell, the texture of the cathode

will be imprinted onto the lithium – through the ASL. This can fracture the ASL. With the

smooth, complaint, polymeric separator coated over the ASL, the ASL is mechanically screened

from the cathode surface and can remain intact. With the ASL ceramic layers intact, the metallic

lithium anode can be protected from reacting with the solvent, leading to increased cycle life.

With the special gel material and pressure the cell can avoid thermal runaway even after the

lithium melts. In Figure 7, the cells are placed in an oven and the temperature is ramped at 5

ºC/min and the difference between the cell and oven temperatures is plotted. Without pressure or

the special gel, cells experience thermal runaway between 130 ºC and 140 ºC. With pressure,

and without the special gel, cells undergo thermal runaway when the lithium melts, at 181 ºC.

The increase in runaway temperature with pressure is due to the reduced surface area of the

lithium. However, when cells with the special compliant polymer gel layer are cycled under

pressure there is no thermal runaway. Even when the lithium melts the cell temperature does not

increase more than about 10 ºC above the oven temperature. After the thermal ramp tests, with

special compliant polymer coated cells taken to 230 ºC, some smooth shiny lithium can still be

found in the cell.

Conclusion: All six anode protection strategies, and the engineered cathode porosity, depicted in

the Future EV Cell roadmap of Figure 1 are being developed to enable Li-S for electric vehicle

battery applications. All told, there are four vacuum deposition processes, and four atmospheric

deposition processes, required to fabricate the cell structure of Figure 1 – all major cell

components, except the solvent, are coated. When employed individually, anode protection

strategies 2 through 5 have been shown to increase cycle life from 50% to 200%, while strategy

1 primarily improves energy density. The cathode, with engineered topology, microstructure and

porosity also, primarily, improves energy density but, compared with the conventional cathode

structure, alone the engineered cathode also increases cycle life. The compliant polymer layer,

when used with pressure, is also

seen to eliminate thermal

runaway. The first execution of

all of these technologies together

has already more than tripled

cycle life, in experimental 350

Wh/kg cell formats, and the cells

are still cycling (Figure 6). As

these new Li-S technologies are

developed further, Sion is

projecting to move from the

conventional Li-S cell (LHS of

Figure 9) to the Future EV Li-S

Cell format depicted in the RHS

of Figure 9.

Figure 9. Projected improvements in Li-S cycle life and specific energy between now and 2016.

Acknowledgements: The information, data, or work presented herein was funded in part by the

Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy, under

Award Number DE-AR0000067.

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