Leland Smith

Opening the window for aqueous electrolytes

Lithium batteries can operate with a safer “water-in-salt” electrolyte [Also see Research Article by Suo et al.] By facilitating ion motion between electrodes, electrolytes help to harness the chemical energy in a battery to produce a current and supply usable electric power. Among liquid electrolytes, there are traditional solutions of salt dissolved in solvents (aqueous or organic) and ionic liquids. Lithium-ion batteries, whether for consumer electronics or electric vehicles, have used nonaqueous liquid electrolytes because of their higher voltage stability. Although aqueous electrolytes would be safer and more environmentally benign, the electrochemical voltage window (1.23 V) precludes the use of high-voltage electrode couples that enable the high energy density of lithium-ion batteries. On page 938 of this issue, Suo et al. (1) change this perception by demonstrating an operating window of 3 V created by increasing the salt concentration to form “water-in-salt” electrolytes.

Silica sol–gel chemistry: creating materials and architectures for energy generation and storage

There is widespread recognition that the use of energy in the twenty-first century must be sustainable. Because of its extraordinary flexibility, silica sol–gel chemistry offers the opportunity to create the novel materials and architectures which can lead to significant advances in renewable energy and energy storage technologies. In this paper, we review some of the significant contributions of silica sol–gel chemistry to these fields with particular emphasis on electrolytes and separators where sol–gel approaches to functionalization and encapsulation have been of central importance. Examples are presented in the areas of dye-sensitized solar cells, biofuel cells, proton exchange membrane fuel cells, redox flow batteries and electrochemical energy storage. Original work is also included for the sol–gel encapsulation of a room temperature ionic liquid to create a solid state electrolyte for electrochemical capacitors. In view of the critical importance of energy and the versatility of the sol–gel process, we expect the sol–gel field to play an increasingly important role in the development of sustainable energy generation and storage technologies.

Sol–gel encapsulated lithium polysulfide catholyte and its application in lithium–sulfur batteries

Lithium polysulfides are the active cathode species in lithium–sulfur batteries. In this work non-hydrolytic sol–gel chemistry is tuned to create a sol that successfully encapsulates lithium polysulfide solutions, forming a solid polysulfide gel. The chemistry of the polysulfide gel is studied using Fourier transform infrared and Raman spectroscopies, which confirm the presence of active lithium polysulfides. This polysulfide gel is incorporated into a solid state lithium sulfur battery and cycled galvanostatically. Electrochemical impedance spectroscopy confirms that the gel has very similar electrochemical properties to the lithium polysulfide solutions from which it was prepared. Discharge capacities as high as 1 mA h cm−2 are obtained for a polysulfide gel cathode that is 1 mm thick.

A high-energy-density quasi-solid-state carbon nanotube electrochemical double-layer capacitor with ionogel electrolyte

This paper reports the fabrication of an electric double-layer supercapacitor that incorporates carbon nanotube electrodes with an ionogel electrolyte based on the sol–gel encapsulation of the ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate. The quasi-solid nature of the ionogel electrolyte enables this supercapacitor to become a solid-state device, which overcomes the limitations of many high-performance supercapacitors, whose liquid-based electrolyte prohibits the realization of truly compact energy storage that is practical for various existing and emerging portable electronics. The device reported here combines the intrinsic high power of an electrochemical double-layer capacitor with the increased energy density associated with the wide voltage window of the ionogel. The device was characterized by a suite of electrochemistry techniques and demonstrated both high power and high energy density.

Synthesis and Properties of a Photopatternable Lithium‐Ion Conducting Solid Electrolyte

One of the important considerations for the development of on-chip batteries is the need to photopattern the solid electrolyte directly on electrodes. Herein, the photopatterning of a lithium-ion conducting solid electrolyte is demonstrated by modifying a well-known negative photoresist, SU-8, with LiClO4 . The resulting material exhibits a room temperature ionic conductivity of 52 µS cm-1 with a wide electrochemical window (>5 V). Half-cell galvanostatic testing of 3 µm thin films spin-coated on amorphous silicon validates its use for on-chip energy-storage applications. The modified SU-8 possesses excellent mechanical integrity, is thermally stable up to 250 °C, and can be photopatterned with micrometer-scale resolution. These results present a promising direction for the integration of electrochemical energy storage in microelectronics.

Carbon-ionogel supercapacitors for integrated microelectronics.

To exceed the performance limits of dielectric capacitors in microelectronic circuit applications, we design and demonstrate on-chip coplanar electric double-layer capacitors (EDLCs), or supercapacitors, employing carbon-coated gold electrodes with ionogel electrolyte. The formation of carbon-coated microelectrodes is accomplished by solution processing and results in a ten-fold increase in EDLC capacitance compared to bare gold electrodes without carbon. At frequencies up to 10 Hz, an areal capacitance of 2.1 pF μm(-2) is achieved for coplanar carbon-ionogel EDLCs with 10 μm electrode gaps and 0.14 mm(2) electrode area. Our smallest devices, comprised of 5 μm electrode gaps and 80 μm(2) of active electrode area, reach areal capacitance values of ∼0.3 pF μm(-2) at frequencies up to 1 kHz, even without carbon. To our knowledge, these are the highest reported values to date for on-chip EDLCs with sub-mm(2) areas. A physical EDLC model is developed through the use of computer-aided simulations for design exploration and optimization of coplanar EDLCs. Through modeling and comparison with experimental data, we highlight the importance of reducing the electrode gap and electrolyte resistance to achieve maximum performance from on-chip EDLCs.

Scaled carbon-ionogel supercapacitors for electronic circuits

Capacitors are ubiquitous in signal processing circuits. Dielectric capacitors based on metal-oxide-semiconductor (MOS) and metal-insulator-metal (MIM) designs are currently the industry standard for on-chip charge storage. By comparison, electric double-layer capacitors (EDLC), or supercapacitors, offer capacitances that are orders of magnitude higher than dielectric capacitors. In this paper we present some early work in fabricating solid-state, on-chip EDLC.