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Energy Storage
Research
The Conn Center has been actively involved in the development of new materials, novel processing techniques, and innovative chemistries for Li-ion batteries, Li-S batteries, solid-state batteries, and flow batteries. The facility is focused on development of low cost and in large scale battery materials with high energy density and high power density. This involves innovation in novel functional nano-materials, low cost processing techniques such as dry manufacturing, rapid screening of battery materials, and modeling of battery performance.
Current strategic plan at the Conn Center has defined short term and long term targets for each battery chemistry.
Lithium-ion Batteries
Lithium ion batteries are widely used in consumer electronic devices such as laptops and cellphones and are currently being considered for automobile applications such as in PEV and EV and stationary storage applications including grid-energy storage. For EV applications, high energy density, cost, and safety are critical and largely determined by choice of electrode materials. Presently, the commercial lithium ion technology is largely limited to cells with gravimetric energy densities of <250 Wh/kg and volumetric energy densities of <650 Wh/L. While the energy densities are not critical for grid storage, volumetric energy densities are often more important for portable electronics and electric vehicles.
There is immense need to push the energy densities to as high as ∼500 Wh/kg and 1,000 Wh/L.
Our current research on anode materials involve insertion reaction materials such as Li4Ti5O12 and conversion reaction materials such as SnO2 nanowires, Si nanowires, MoO3 nanowires, Li2MoO4 and Li2MoO2 nanowires, etc. whereas cathode materials include Li2MnO3 nanowires, layered and spinel Lithium Nickel Manganese Cobalt Oxide (NMC) (both Ni and Mn rich) materials and olivine LiFePO4. Our team has introduced a simple, generic design for stable, high-capacity anode materials consisting of hybrid structures involving metal nanoclusters supported on metal oxide nanowires.
The overall target defined by our strategic plan in Li-ion battery research is to reduce the cost for the full cell from $500/kWh to $125/kWh with the increase of energy density from 100 Wh/kg to 250 Wh/kg for over 1000 deep cycles.
Lithium-Sulfur Batteries
Currently available battery technologies based on Li-ion batteries are limited with the energy density (~250 mAh/g for cathodes). Li-S is an emerging chemistry that could provide energy density (up to 1650 mAh/g for cathode) and can enable meeting US DOE targets for energy density of 400 Wh/kg at C/3 rate. The development of advanced durable and high capacity Li-S battery technology can revolutionize both automotive and ubiquitous energy storage applications. Other areas of interest could include renewable energy storage, space exploration, and grid stabilization etc. However, the practical utility of lithium-sulfur batteries is hampered by persistent problems such as poor cycle life and shelf-life due to the poor electronic conductivity of sulfur, diffusion of dissolved polysulfide species from the sulfur cathode to the lithium-metal anode through the separator, and instability of the lithium-metal anode.
We have introduced new strategies to improve the aforementioned problems by (i) engineering sulfur cathode with novel supports and coatings (ii) using pre-lithiated high energy density nanostructured materials as anode materials and (iii) using special liquid-, and ionic-liquid-based solid polymer electrolytes and additives. Current research on Li-S batteries include the optimization of full cells based on engineered sulfur cathodes and pre-lithiated nanowire based anodes.
The overall target defined by our strategic plan in Li-S battery research is to reduce the cost for the full cell from $500/kWh to $125/kWh with the increase of energy density from 100 Wh/kg to 250 Wh/kg for over 1000 deep cycles.
Above figure summarizes the proposed strategies to develop efficient Li-S batteries. The cathode is prepared using carbonized PAN based spun fibers loaded with sulfur from melt and then coated with TiO2 nanoparticles from solution. The cathode material is pressed onto a stainless steel current collector through a PVDF/Acetylene black conducting buffer layer. With this novel architecture, we have been able to fabricate sulfur cathodes to sustain the discharge capacity exceeding 1000 mAh/g over 50 cycles. We have also developed prelithiated MoO3 nanowires to replace Li metal.
Sodium-ion batteries
Despite the recent successes in Li-based battery technology, there are increasing concerns regarding the sustainability of lithium sources due to their limited availability and consequent price increase. As a result, recent research has focused on alternative energy storage systems. Sodium ion batteries (SIBs) are considered as one of the best candidate power sources because sodium is widely available and exhibits similar chemistry to that of Lithium Ion Batteries making SIBs promising next-generation alternatives. Sodiated layer transition metal oxides, phosphates and organic compounds have been introduced as cathode materials for SIBs. On the anode side, carbonaceous materials, transition metal oxides (or sulfides), and intermetallic and organic compounds have been tried as anodes for SIBs. Apart from electrode materials, suitable electrolytes, additives, and binders are equally important for the development of practical SIBs. Despite developments in electrode materials and other components, there remain several challenges, including cell design and electrode balancing, in the application of sodium ion cells. Our research on Na-ion battery involves optimization of sodiated NMC based cathodes and nanostructured anodes.
The overall target defined by our strategic plan in Na-ion battery research is to lower the cost down to $250/kWh at pack level with 13.5 kWh energy with a 4-hr discharge time.
Solid State Batteries
All-solid-state Li batteries, replacing liquid organics with ceramic solid-state electrolytes, have the potential to solve the crucial problems (i.e. safety issues, low energy density) associated with currently conventional Li-ion batteries. Unlike liquid electrolyte, inorganic solid electrolytes have much better thermal stability to prevent potential burning or explosion risks. In addition, solid-state batteries also allow for the use of metallic Li as anode material, which has the highest specific capacity (3,860 mAh/g) and low electrochemical potential. For all-solid-state batteries, solid electrolytes undoubtedly play critical importance to determine the battery performance. Ideal solid electrolytes are expected to exhibit excellent ionic conductivities (>1mS cm-1), good thermal and air stability, electrochemical stability with electrodes, low cost and scalability. Popular solid electrolytes include garnet, perovskite, and thio-phosphate, etc. Our research in this area include optimization of oxide and sulfide based solid state electrolytes for Li-ion, Li-S, and Na-ion batteries.
The overall target defined by our strategic plan in solid state battery research is to produce 10 kWh Energy batteries with cost of $1000/kW and $300/kWh.
Flow Batteries
A flow battery is an electrochemical storage device equivalent to both a conventional battery and a fuel cell. In conventional batteries energy is stored in the electrode material whereas in flow batteries energy is stored in the electrolyte. A flow battery is a type of rechargeable battery where chemical components are dissolved in liquids contained within the system and separated by a membrane. One of the biggest advantages of flow batteries is that they can be recharged instantly by replacing the electrolyte liquid, while simultaneously recovering the spent material for re-energization. Rechargeable redox flow batteries (RFB) are considered as promising candidates for medium and large-scale stationary energy storage applications. RFB is a promising technology due to its ability of decoupling of power output and energy storage capacity and typically use electrodes made of porous carbon structures (e.g. felts, cloths, or papers) fixed in the electrochemical device. In a different variation of flow batteries, slurry electrodes are made by suspending solid, electronically conductive particles (e.g. carbon nanotubes) in an electrolyte are used.
Current research at Conn center is geared towards the development of flow batteries for grid scale energy storage applications. This includes the fabrication of novel flow cell designs and electrochemical testing. Electrolyte cost is a road block in the commercialization of flow batteries. To improve on that aspect of flow battery research, we are working to increase energy density by utilizing redox couples with high potential difference in an aqueous environment. Currently we are studying several redox couples for such flow batteries.
The overall target defined by our strategic plan in flow battery research is to produce 10 kW flow batteries with 80kWh with 8 hr. charging achieving 80kWh energy.
Dry Manufacturing of Electrodes
Commercial Li-ion battery electrodes are manufactured by casting a slurry onto metallic current collectors. The slurry containing active material, conductive carbon, and binder undergoing various manufacturing processes such as coating and drying have been identified as a major obstacle in meeting cost effective electrode fabrication targets. In order to meet the targets, it is required to (i) lower the electrode processing cost associated with the costly organic solvent and primary solvent drying time; (ii) substantially increase the electrode thicknesses to meet the power level while preserving the power density; and (iii) reduce the formation time of the solid electrolyte interface (SEI) layer. Thicker electrode designs are essential to achieve the necessary capacity targets for electrodes. As the thickness increases, challenges exist for engineering conductivity and electrode-electrolyte contact. Gradual control of porosity and particle size with thickness is required to maintain improved electronic conductivity of the film and reduced diffusion length scales for intercalating ions within the electrode materials. Electrodes manufactured with dry particles coated on current collectors represent the ideal manufacturing process, thereby eliminating solvents and other disadvantages associated with them.
Our research at Conn Center is focused on the manufacture of electrode materials for lithium-ion batteries using an atmospheric plasma stream in carrying, heating, and directing current collector and electrode materials for deposition on thin sheet substrates. We plan to develop a low-cost, environmentally friendly, dry manufacturing process to fabricate battery electrodes replacing the conventional wet-slurry based manufacturing process.