Aqueous lithium ion batteries (ALIBs) exhibit great potential to reduce the cost and improve the safety of rechargeable energy storage technologies. Lithium iron phosphate (LFP) cathodes have become a material of choice for many conventional, high power LIBs. However, experimental studies on LFP in aqueous lithium (Li) ion electrolytes are limited. Here, results of systematic studies are shown where it is demonstrated that the Li salt concentration of the aqueous electrolyte can significantly improve discharge capacity retention while minimally impacting rate capability, for electrodes made with a typical commercial sub-micron sized LFP powder. Based on the postmortem analysis and the results of electrochemical characterization it is proposed that undesirable side reactions of aqueous electrolytes with LFP induce electrochemical separation of individual particles within the electrode, leading to the observed capacity fading. Increasing the salt concentration in aqueous solutions effectively reduces the concentration of water molecules in the electrolyte, which are mostly responsible for these undesirable side reactions. Similar trends observed with other cathode materials suggest that the use of concentrated aqueous electrolyte solutions offers an effective route to improve stability of aqueous Li ion batteries.

Lithium–metal fluoride (MF) batteries offer the highest theoretical energy density, exceeding that of the sulfur–lithium cells. However, conversion-type MF cathodes suffer from high resistance, small capacity utilization at room temperature, irreversible structural changes, and rapid capacity fading with cycling. In this study, the successful application of the approach to overcome such limitations and dramatically enhance electrochemical performance of Li–MF cells is reported. By using iron fluoride (FeF₂) as an example, Li–MF cells capable of achieving near-theoretical capacity utilization are shown when MF is infiltrated into the carbon mesopores. Most importantly, the ability of electrolytes based on the lithium bis(fluorosulfonyl)imide (LiFSI) salt is presented to successfully prevent the cathode dissolution and leaching via in situ formation of a Li ion permeable protective surface layer. This layer forms as a result of electrolyte reduction/oxidation reactions during the first cycle of the conversion reaction, thus minimizing the capacity losses during cycling. Postmortem analysis shows the absence of Li dendrites, which is important for safer use of Li metal anodes. As a result, Li–FeF₂ cells demonstrate over 1000 stable cycles. Quantum chemistry calculations and postmortem analysis provide insights into the mechanisms of the passivation layer formation and the performance boost.

Development of sulfur cathodes with 100% coulombic efficiency (CE) and good cycle stability remains challenging due to the polysulfide dissolution in electrolytes. Here, it is demonstrated that electrochemical reduction of lithium bis(fluorosulfonyl)imide (LiFSI) based electrolytes at a potential close to the sulfur cathode operation forms in situ protective coating on both cathode and anode surfaces. Quantum chemistry studies suggest the coating formation is initiated by the FSI(-F) anion radicals generated during electrolyte reduction. Such a reduction additionally results in the formation of LiF. Accelerated cycle stability tests at 60 °C in a very simple electrolyte (LiFSI in dimethoxyethane with no additives) show an average CE approaching 100.0% over 1000 cycles with a capacity decay less than 0.013% per cycle after stabilization. Such a remarkable performance suggests a great promise of both an in situ formation of protective solid electrolyte coatings to avoid unwanted side reactions and the use of a LiFSI salt for this purpose.

Lithium insertion into sulfur confined within 200 nm cylindrical inner pores of individual carbon nanotubes (CNTs) was monitored in situ in a transmission electron microscope (TEM). This electrochemical reaction was initiated at one end of the S-filled CNTs. The material expansion during lithiation was accommodated by the expansion into the remaining empty pore volume and no fracture of the CNT walls was detected. A sharp interface between the initial and lithiated S was observed. The reaction front was flat, oriented perpendicular to the confined S cylinder, and propagated along the cylinder length. Lithiation of S in the proximity of conductive carbon proceeded at the same rate as the one in the center of the pore, suggesting the presence of electron pathways at the Li₂S/S interface. Density of states calculations further confirmed this hypothesis. In situ electron diffraction showed a direct phase transformation of S into nanocrystalline Li₂S without detectable formation of any intermediates, such as polysulfides and LiS. These important insights may elucidate some of the reaction mechanisms and guide the improvements in the design of C–S nanocomposites for high specific energy Li–S batteries. The proposed use of conductive CNTs with tunable pore diameter as cylindrical reaction vessels for in situ TEM studies of electrochemical reactions proved to be highly advantageous and may help to resolve the ongoing problems in battery technology.

One of the most challenging problems in the development of lithium–sulfur batteries is polysulfide dissolution, which leads to cell overcharge and low columbic efficiency. Here, we propose the formation of a thin conformal Li-ion permeable oxide layer on the sulfur-carbon composite electrode surface by rapid plasma enhanced atomic layer deposition (PEALD) in order to prevent this dissolution, while preserving electrical connectivity within the individual electrode particles. PEALD synthesis offers a fast deposition rate combined with a low operating temperature, which allows sulfur evaporation during deposition to be avoided. After PEALD of a thin layer of aluminium oxide on the surface of electrode composed of large (ca. 10 μm in diameter) S-infiltrated activated carbon fibers (S-ACF), significantly enhanced cycle life is observed, with a capacity in excess of 600 mA·h·g⁻¹ after 300 charge–discharge cycles. Scanning electron microscopy (SEM) shows a significant amount of redeposited lithium sulfides on the external surface of regular S-ACF electrodes. However, the PEALD alumina-coated electrodes show no lithium sulfide deposits on the fiber surface. Energy dispersive spectroscopy (EDS) studies of the electrodes’ chemical composition further confirms that PEALD alumina coatings dramatically reduce S dissolution from the cathodes by confining the polysulfides inside the alumina barrier.

As electrical energy storage and delivery devices, carbon-based electrical double-layer capacitors (EDLCs) have attracted much attention for advancing the energy-efficient economy. Conventional methods for activated carbon (AC) synthesis offer limited control of their surface area and porosity, which results in a typical specific capacitance of 70–120 F g⁻¹ in commercial EDLCs based on organic electrolytes and ionic liquids (ILs). Additionally, typical ACs produced from natural precursors suffer from the significant variation of their properties, which is detrimental for EDLC use in automotive applications. A novel method for AC synthesis for EDLCs is proposed. This method is based on direct activation of synthetic polymers. The proposed procedure allowed us to produce ACs with ultrahigh specific surface area of up to 3432 m² g⁻¹ and volume of 0.5–4 nm pores up to 2.39 cm³ g⁻¹. The application of the produced carbons in EDLCs based on IL electrolyte showed specific capacitance approaching 300 F g⁻¹, which is unprecedented for carbon materials, and 5–8% performance improvement after 10 000 charge–discharge cycles at the very high current density of 10 A g⁻¹. The remarkable characteristics of the produced materials and the capability of the fabricated EDLCs to operate safely in a wide electrochemical window at elevated temperatures, suggest that the proposed synthesis route offers excellent potential for large-scale material production for EDLC use in electric vehicles and industrial applications.

The ability to quickly store and deliver a significant amount of electrical energy at ultralow temperatures is critical for the energy-efficient operation of high altitude aircraft and spacecraft, exploration of natural resources in polar regions and extreme altitudes, and astronomical observatories exposed to ultralow temperatures. Commercial high-power electrochemical capacitors fail to operate at temperatures below –40 °C. According to conventional wisdom, mesoporous electrochemical capacitor electrodes with pores large enough to accommodate fully solvated ions are needed for sufficiently rapid ion transport at lower temperatures. It is demonstrated that strictly microporous carbon electrodes with much higher volumetric capacitance can be efficiently used at temperatures as low as –70 °C. The critical parameters, with respect to electrolyte properties and electrode porosity and microstructure, needed for achieving both rapid ion transport and efficient ion electroadsorption in porous carbons are discussed. As an example, the fabrication of an electrochemical capacitor with an outstanding performance at temperatures as low as –60 and –70 °C is demonstrated. At such low temperatures the capacitance of the synthesized electrodes is up to 123 F g⁻¹ (≈76 F cm⁻³), which is 50–100% higher than that of the most common commercial electrochemical capacitor electrode at room temperature. At –60 °C selected cells based on ≈0.2 mm electrodes exhibited characteristic charge–discharge time constants of less than 9 s, which is faster than the majority of commercial devices at room temperature. The achieved combination of high energy and power densities at such ultralow temperatures is unprecedented and extremely promising for the advancement of energy storage systems.

The detonation nanodiamond is a versatile low-cost nanomaterial with tunable properties and surface chemistry. In this work, it is shown how the application of nanodiamond (ND) can greatly increase the performance of electrochemically active polymers, such as polyaniline (PANI). Symmetric supercapacitors containing PANI-ND nanocomposite electrodes with 3–28 wt% ND show dramatically improved cycle stability and higher capacitance retention at fast sweep rate than pure PANI electrodes. Contrary to other PANI-carbon nanocomposites, specific capacitance of the selected PANI electrodes with embedded ND increases after 10 000 galvanostatic cycles and reaches 640 F g⁻¹, when measured in a symmetric two-electrode configuration with 1 M H₂SO₄ electrolyte. The demonstrated specific capacitance is 3–4 times higher than that of the activated carbons and more than 15 times higher than that of ND and onion-like carbon (OLC).