While Lithium-ion batteries (LIBs) have been a popular choice for electric vehicles (EVs), their limitations are driving the search for better solutions in the EV battery segment. Rising costs, concerns about raw material availability, and a desire for even higher performance are mainly driving this exploration of alternative battery chemistries and systems.  

One promising alternative is the lithium-sulfur (Li-S) EV battery.

Li-S batteries currently comprise a small portion of the EV battery market. However, the market size is expected to reach up to $15 billion, or 10% of the total global market, by 2028. 

In this blog post, we will explore why lithium-sulfur batteries can be environmentally friendly alternatives to Lithium-ion batteries and highlight the existing research developments for sustainable EV battery applications. 

Challenges of Lithium-ion EV Battery: Need for Alternatives

De-carbonization in electricity generation can help significantly reduce CO2 emissions in the ongoing electrification of transport systems, including electric vehicles (EVs). Despite LIBs  being a popular choice for EV batteries,  there are several challenges that need urgent intervention:

Cathode Costs and Degradation Issues

The high energy density and cyclability of Lithium-ion batteries (LIBs) make them attractive for EVs. However, their cost, often around $176/kWh, poses a significant challenge. This high price is primarily due to the expensive materials used in the cathode, which is a must-have component for the batteries. 

The high cost of ternary oxides in NMC cathode materials has led to a shift towards nickel-rich and cobalt-less cathodes primarily derived from expensive transition metals like Co, Ni, and Mn. This hierarchy followed from NMC111 to NMC811 and then to nickel-rich LiNi1-xMxO2 layered oxides. 

While nickel-rich cathodes offer some advantages, they remain expensive and suffer from degradation issues, hindering their widespread adoption.

SEI and Safety Concerns

An anode’s electrochemical potential is typically above the organic electrolyte’s lowest unoccupied molecular orbital, forming a solid electrolyte interphase (SEI). Due to volume expansion, these SEIs can be susceptible to repeated breakdown and regrowth, and lithium dendrite can accompany growth, leading to cell short-circuiting or fire.

Limited Raw Material Availability

The increasing demand for lithium, cobalt, nickel, and other key metals could lead to supply chain constraints and price volatility. Sustainable sourcing and potential recycling efforts are urgently needed.

Performance Limitations

While improving, LIBs can still be affected by temperature extremes, impacting range and charging time. So, further advancements in energy density are required to increase the range of the EV battery.

Li-S Batteries: Are They Better Alternative for EV Battery?

Owing to the drawbacks of LIB batteries, there’s a clear need to find better solutions and continue research and development in this field. 

Lithium-sulfur (Li-S) batteries emerge as a potential alternative, with a high energy density of 2,600 Wh kg-1, which is 5x higher than conventional LIB batteries.

Let us review a few factors contributing to its potential: 

Abundance of Materials

Sulfur, a plentiful, easily accessible, and cost-effective material, reduces the need for scarce and geographically concentrated minerals like cobalt in EV battery applications.

Sion Power and Oxis Energy are repurposing abundant and low-cost sulfur for lithium-sulfur battery cathodes, reducing their reliance on new sulfur sources and minimizing the environmental impact of mineral extraction. 

Thus, they avoid waste and promote sustainable practices.

Reduced Heavy Mining 

The cathode in Li-S batteries is predominantly made of elemental sulfur, which may be supplied from various sources, including waste streams from petroleum refining and natural gas processing. This significantly reduces the need for large-scale mining.

Lithium-sulfur battery production relies on sulfur from waste streams from petroleum refining and natural gas processing industries. For example, the Athabasca oil sands in Alberta, Canada, contain enough sulfur to potentially yield over 100 million tons upon extraction and refining.

Higher Theoretical Energy Density

As noted above, Li-S batteries offer higher theoretical energy density than Lithium-ion batteries. This can potentially enable longer driving ranges and smaller battery pack sizes, reducing material consumption and lowering environmental footprint.

Improved Safety

In 2019, a Tesla Model S collided with a high-speed vehicle in Austria, causing a Lithium-ion battery pack to catch fire. Emergency services responded with specialized fire trucks and hazardous materials teams due to the potential risk of toxic fumes and environmental contamination from the burning battery pack containing heavy metals.

On the other hand, Sulfur, unlike some Lithium-ion cathode materials, is non-toxic and environmentally friendly, potentially reducing the risk of environmental contamination due to battery leakage or improper disposal.

Recyclability 

The Li-S batteries, primarily composed of lithium, sulfur, and carbon, may be easier to recycle and recover valuable materials than complex metal mixtures in Lithium-ion cathodes.

Researchers suggest that a recycling process for Li-S batteries could recover up to 93% of lithium and sulfur content. 

This is significantly higher than the typical recovery rates of 50% to 70% for valuable metals like cobalt and nickel.

EV Battery: Some Drawbacks of  Lithium-sulfur Batteries and Their Solutions

Now, let’s go over some limitations of Li-S batteries and how to overcome them:  

Closing the Gap: From Theoretical to Practical 

There is a significant gap between the theoretical capacity of lithium-sulfur batteries and the practically targeted capacity. This stems from challenges such as: 

• Polysulfide shuttle
• Lithium dendrite growth 
• Poor cathode conductivity
• Slow conversion kinetics due to their electrically nonconductive and inferior ionic conductivity in EV battery applications

Researchers are pursuing multiple innovative strategies to bridge the gap between theoretical and practical Li-S battery performance. 

• These include trapping polysulfides within intricate carbon structures or chemically binding them to prevent shuttling. 
• Efforts also focus on preventing lithium dendrite growth through electrolyte adjustments and protective anode layers. 
• Scientists are enhancing cathode conductivity with conductive carbon networks and polymers while exploring catalysts to speed up sluggish reaction rates. 

Insulating Materials and Li-S Precipitates

The poor rate capability in lithium-sulfur batteries is another major roadblock to its mainstream adoption. This occurs primarily due to the insulating nature of sulfur and reduction products and the inhomogeneous Li2S/Li2S2 precipitates that can form an inert layer on the cathode surface, causing degradation of electrochemical performance. 

To solve this, using physical confinement in carbon host/sulfur composite cathodes and polar carbon hosts with heteroatom-doping have been attempted to address these issues.

Electrolyte Interaction and Electrode Stability

Electrolyte wettability on electrodes is crucial, especially under lean electrolyte conditions. Carbon host design must consider porosity, sulfur distribution, and chemical and mechanical stability due to volume expansion and Li2S formation under high-sulfur-loading conditions. 

Improving the cyclability and rate capability of lithium-sulfur batteries requires considering other vital parameters, such as polysulfide dynamics, in addition to cathode design.

To Sum It Up

High-capacity sulfur chemistry in lithium-sulfur batteries has the potential to become a leading champion in the “Beyond Li-ion” battery in the EV battery market, with the potential for over 500 WHkg-1 in the near future. 

However, technical bottlenecks include high sulfur loading in the cathode and long lifespans. There is a significant gap between laboratory and industrial lithium-sulfur batteries, with challenges such as slow polysulfide reaction kinetics, inert electronic/ionic characteristics, and polysulfide shuttle. A systematic approach is needed to explore effective strategies.

All-solid-state Li-S batteries are the ultimate goal for large-scale practical applications. Soft sulfides and halide-based electrolytes can help to solve further challenges due to their ease of manufacture and high performance. Large-scale air-stable synthesis processes should also be explored for low-cost commercial applications.