Li-S Battery

 A Li-S cell works based on the following conversion reaction: 2Li + S ↔ Li2S

Reference:  doi: 10.3389/fenrg.2019.00123

Example Li-S battery BOM

Benefits of Li-S batteries

  • S is one of the richest elements in earth crust
  • S is a light element, which delivers a high theoretical capacity of ∼1,675 mAh/g,
  • Pouch cell, as a double side anode, the energy capacity should be at least 10 mAh/cm2, which is based on the loading of sulfur is 5 mg/cm2, and specific capacity is 1,000 mAh/g. 
  • High rate capabilities of more than 10 C (1 C = 1,675 mA/g) and long life-span over 1,000 cycles have been reported in many research papers based on coin cells
  • Discharge voltage of 2.15 V, the theoretical gravimetric energy density of a Li-S cell is ∼2,510 Wh/kg 
  • Coulombic efficiency is commonly reported to be more than 99% in literature; where LiNO3 additive is used; the more LiNO3 used, the higher the Coulombic efficiency and longer cycle life,

Challenges

  • polysulfides dissolution->shuttle effect
  • poor cyclability and lithium anode corrosion, 
  • poor electronic conductivity
  • significant volume change during cycling

Solutions to polysulfides shuttle effect

  • use of coating layers on electrode and separator
  • interlayers between cathode and separator

Solutions to Lithium dendrite effect

  • protective layers on lithium metal, artificial SEI layers
  • constructing 3D current collectors; porous conductive foam, hollow carbon fiber cloth, could be a good choice after optimization for lithium plating
  • solid electrolytes
  • electrolyte additives

anode

Li specific capacity of 3,860 mAh/g and the lowest potential of −3.04V (vs. standard hydrogen electrode)

A theoretical areal capacity: about 1mAh/cm^2 with 5um Li; 5mg/cm^2 S->8.4mAh/cm^2; real Li thickness should be at least 3X min. thickness

Corrosion and pulverization of Li anode-> lose electric contact to Li foil

To mitigate the above problem, building artificial SEI layers, using modified electrolytes, and constructing 3D current collectors

 cathode

Cathode composition

Materials in a cathode include a mixture of the commercial carbon materials (e.g., acetylene black, CNT, graphene, etc.), sulfur, and binder (3–6 wt.%). S-loading: be 75–78 wt.% (1mg additives:3mg S) to achieve specific energy of 400 Wh/g , for Li-S pouch cells. Upto 50% for coin cell (1 mg sulfur and 1 mg additives (including conductive agent and binder) )

Improvement the electronic conductivity of sulfur composites: carbon nanotubes, graphene, porous carbon, and hybrid carbon. Many carbon materials with the high specific surface area may not be suitable for commercialization due to the additional side reactions, low volumetric capacities, need more binder, and difficulties in the slurry casting process of cathodes.

In Li-ion battery industry, specific surface area of active materials is generally <1m^2/g and 1-3wt% of binder in cathode

suppression the polysulfides shuttle effect by absorbents, and 

promotion reaction kinetics of sulfur species using catalysts

interlayer and separators

There are two main pathways for preparing the multi-functional separators:

  • modify the composition and structure of separator to improve the ionic selectivity and suppress the diffusion of lithium polysulfides
  • add a blocking layer between the separator and cathode/anode to inhibit the transport of lithium polysulfides.
  • porous activated carbon nanofiber films were demonstrated to adsorb the lithium polysulfides and restrain their diffusion, thus suppressing the shuttle effect.
  • separators modified with polypyrrole nanotubes
  • novel separator of graphdiyne (GDY)-modified polyimide (PI) separator
  • Placing an adsorption interlayer between the cathode and separator not only enhances the conductivity and the utilization of cathode but also relieves the shuttle effect of lithium polysulfides . Adsorbents such as Al2 O3 , TiO2, and MnO2 are widely reported to bind polysulfides through physical and/or chemical interaction

Electrolyte

LiN(SO2 CF3 )2 (LiTFSI) lithium salt in mixtures of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) has been the most used liquid electrolyte for Li-S batteries. 

Ionic liquid is a good choice for the electrolyte of Li-S batteries, owing to their incredible properties including non-volatility, high thermal stability, and high ionic conductivity 

For Li-ion batteries, the regular dose of electrolyte is ∼3 g/Ah in the industrial production process 

The solid electrolyte is an idea choice for the capability of blocking the shuttle effect of polysulfides

A lithium bis(fluorosulfonyl)imide/poly(ethylene oxide) (LiFSI/PEO) polymer used as solid electrolyte for Li-S batteries, which can effectively suppress the shuttle effect of polysulfides (Ref: doi: 10.1021/acs.jpclett.7b00593

Source:doi: 10.1021/acs.jpclett.7b00593