Source: SolidEnergy. Click to enlarge.

The core of SolidEnergy’s technology is a Solid Polymer Ionic Liquid (SPIL) electrolyte, originally developed at and licensed from MIT.

The SolidEnergy prototypes combine a cathode (SPIL can work with a range of cathode materials); the novel electrolyte; and a solid-polymer-coated lithium metal anode. The electrolyte combines ionic liquid and liquid polymer to provide both the safety and wide temperature capability required for advanced batteries, while the solid-polymer-coated lithium anode boosts energy density and cycle life. In addition, Lithium dendrite- suppressing additives boost safety.

The polymer and ionic liquid both have low vapor pressure and are safe up to 300 °C, while the solid polymer coating with dendrite-suppressing additives prevent dendrite growth.

As Dr. Hu and his colleagues at MIT noted in a 2011 paper in the Journal of Power Sources:

The use of conventional lithium-ion batteries in high temperature applications (>50 °C) is currently inhibited by the high reactivity and volatility of liquid electrolytes. Solvent-free, solid-state polymer electrolytes allow for safe and stable operation of lithium-ion batteries, even at elevated temperatures. Recent advances in polymer synthesis have led to the development of novel materials that exhibit solid-like mechanical behavior while providing the ionic conductivities approaching that of liquid electrolytes.

—Hu et al.

That paper, and the subsequent patents awarded to MIT and licensed by SolidEnergy, involve the use of a solid graft copolymer electrolyte (GCE). A block copolymer consists of two chemically dissimilar polymers covalently bonded end-to-end. Under certain conditions, the different polymer blocks undergo micro-phase separation into periodically spaced nanoscopic domains. The morphology can be controlled by the overall composition of the copolymer. When a lithium salt is added, the polymer becomes a solid electrolyte, behaving mechanically like a rubbery solid but still possessing high ionic conductivity: a block copolymer electrolyte (BCE).

In a 2004 paper in the Journal of Power Sources, Professor Sadoway of MIT observed that electrical conductivity of BCEs varied inversely with the glass transition temperature of one of the block components.

In an effort to raise the electrical conductivity to even higher values, for the choice of secondary block we turned to poly(dimethyl siloxane) (PDMS) with its T_g of −123 ˚C, substantially lower than that of any of the alkyl methacrylates in the previous study. To exploit this chemistry we chose to generate a microphase-separating graft copolymer consisting of a backbone of poly(oxy-ethylene)9 methacrylate (POEM) and long side chains of PDMS. POEM-g-PDMS with 70wt.% POEM and doped with lithium triflate proved to be a rubbery solid exhibiting electrical conductivity nearly equivalent to that of POEM homopolymer, a viscous liquid at room temperature. Termed a graft copolymer electrolyte (GCE), this material was prepared by a facile free-radical synthesis method that is industrially scalable.

—Sadoway (2004)

SolidEnergy’s Intellectual Property (IP) strategy focuses on the novel solid electrolyte, Li-metal anode, interface, packaging, and processing of the battery. SolidEneregy is collaborating with A123 Venture Technologies, a Massachusetts-based technology incubator.

Resources

• Qichao Hu, Solid Energy presentation at NREL Industry Growth Forum 2013

• Qichao Hu, Sebastian Osswald, Reece Daniel, Yan Zhu, Steven Wesel, Luis Ortiz, Donald R. Sadoway (2011) “Graft copolymer-based lithium-ion battery for high-temperature operation,” Journal of Power Sources, Volume 196, Issue 13, Pages 5604-5610, doi: 10.1016/j.jpowsour.2011.03.001

• Patrick E. Trapa, You-Yeon Won, Simon C. Mui, Elsa A. Olivetti, Biying Huang, Donald R. Sadoway, Anne M. Mayes, and Steven Dallek (2005) “Rubbery Graft Copolymer Electrolytes for Solid-State, Thin-Film Lithium Batteries” J. Electrochem. Soc. 152(1): A1-A5 doi: 10.1149/1.1824032

• Donald Sadoway (2004) “Block and graft copolymer electrolytes for high-performance, solid-state, lithium batteries” Journal of Power Sources Volume 129, Issue 1, Pages 1-3 doi: doi:10.1016/j.jpowsour.2003.11.016