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Effective utilization of active materials in microbatteries can be enhanced by rational design of the electrodes. There is an increasing trend of using 3 D electrodes that are coated in nanosized active materials to boost both energy and power densities. This article focuses on the fabrication of 3 D electrodes based on monolithic carbon foams coated in over-stoichiometric NbO2 nanoparticles. The electrodes exhibit remarkable energy and power densities at various current densities when tested in lithium microbatteries. An areal capacity of around 0.7 mAh cm−2 and energy density up to 45 mWh cm−3 have been achieved. More than half of the areal capacity can be accessed at a current density of about 11 mA cm−2, with the corresponding energy and power densities being 21 mWh cm−3 and 1349 mW cm−3. These values are comparable to those of microsupercapacitors containing carbon and MnO2 nanomaterials. Furthermore, the electrochemical reversibility improves progressively upon cycling along with substantial increase in the charge transfer kinetics of the electrode. Based on impedance analyses almost a fourfold decrease in the charge transfer resistance has been observed over 25 cycles. Such enhancement of the electronic properties of NbO2 can account for the high electrochemical rate performance of the 3 D electrodes. [ABSTRACT FROM AUTHOR]

High charge and discharge rates are desired properties for Li-ion batteries of both macro- and micro-scale (i.e. a footprint area of < 1 mm 2 ). Under these conditions, a rise of the cell temperature can take place, leading to performance limitations and safety issues. Investigations of thermal effects in battery cells provide possible explanations of the limiting factors of cell performance and can suggest improvements. We present here extensive simulations with a fully coupled 3D thermal-electrochemical model of 3D microbatteries (3D-MBs) using Finite Element Methodology (FEM). 3D-MB architectures comprising pillar shaped, plate shaped and concentric electrode arrangements are simulated, using LiCoO 2 and graphite as electrodes and solid polymer electrolytes with LiTFSI salt. Sensitivity analysis of the electrolyte diffusion coefficient, depending on the C-rate, is used to benchmark the performance of these 3D-MB cells. FEM simulations of the 3D-MB during operation provide a complete 3D time-dependent description of the thermal behavior of the cells. Temperature gradients in the cell highlight critical regions which are likely causing performance bottlenecks and safety hazards. The simulations clearly demonstrate that the highest heat sources appear near the regions with most active charge transfer processes, thereby providing insights for optimization of the cell geometry in terms of both performance and safety. [ABSTRACT FROM AUTHOR]

We here present finite element methodology simulations of current distribution, materials utilization and electrochemical cell behavior of a rechargeable Li-ion three dimensional microbattery (3D MB) with ‘concentric’ architecture, i.e., with one electrode material deposited through filling a 3D structured pillar template of the other electrode. The electrode materials modelled are LiCoO 2 and lithiated graphite, respectively, where thin films of LiCoO 2 are coated onto 3D-structured current collectors. Time dependent simulations of a range of template pillar heights ( h ) and interpillar distances ( d ) have been performed, and the ionic conduction pathways resulting from the different 3D MB concentric architecture design are analyzed. The dynamics of the discharge curves, Li-ion concentration and concentration gradient for each cell design are compared between the different structures and put into context of the state of charge in the electrodes during one full charge/discharge cycle. It is shown that the architecture with the shortest interpillar distance ( d = 10 μm) provides higher capacity per footprint area without displaying underutilization of the active material. Higher pillars, on the other hand, result in higher areal capacity, and an optimum height is considered to be ca. 70 μm by balancing the active material utilization in the electrodes and the effective battery volumetric usage. Moreover, it is shown that there are significant differences in terms of cell capacity and power capabilities depending on if the anode or the cathode is coated onto the pillar current collectors. [ABSTRACT FROM AUTHOR]

Abstract: Porous LiMn2O4 thick-film cathodes for Li-ion microbatteries are realized by laser-printing. The porous structure of the printed composite cathode consisting of active powder, binder, carbon black and graphite enables ionic and electronic transport through 50–60 μm thick electrodes due to its high intrinsic active surface area. In order to further improve the cycle stability and capacity retention of the laser-printed thick-film cathodes for discharging rates up to 1 C, laser-printed thick films are first calendered in a press and then structured using ultrafast femtosecond-laser radiation in order to form three-dimensional (3D) cathode architectures. It is shown that calendered/laser structured cathodes in the form of rectangular 3D grids exhibit discharge capacity retention of 68% at a 1 C rate, while calendered but unstructured cathodes retain only about 45% of their initial capacity at the same discharge rate. Overall, the improved discharge capacity retention and reduced degradation during later cycles can be attributed to the combination of increased electrical contact with shortened Li-ion pathways. [Copyright &y& Elsevier]

Abstract: Poly(acrylonitrile) films were deposited electrochemically onto vitreous carbon electrodes from air-saturated acrylonitrile. Electrodeposition on gold was investigated with the use of an electrochemical quartz crystal microbalance (EQCM). It was found that the onset of deposition coincided with the reduction of dissolved oxygen at the electrode surface, indicating that the polymerisation reaction is initiated by the nucleophilic superoxide anion. The chemical composition of the film was confirmed by Raman spectroscopy. Such electrodeposited polymer films may have applications as polymer electrolytes for 3D microbattery applications. [Copyright &y& Elsevier]

Reliable integration of thin film solid-state polymer electrolytes (SPEs) with 3D electrodes is one major challenge in microbattery fabrication. We used initiated chemical vapor deposition (iCVD) to produce a series of nanoscale copolymer films comprising hydroxyethyl methacrylate and ethylene glycol diacrylate. Conformal copolymer coatings were applied to a variety of patterned 3D electrodes and subsequently converted into ionic conductors by lithium salt doping. Broad tunability in ionic conductivity was achieved by optimizing the copolymer cross-linking density and matrix polarity, resulting in a room-temperature conductivity of (6.1 ± 2.7) × 10 -6 S cm -1 , the highest value reported for conformal, nanoscale SPEs.

3D microbatteries (3D-MBs) impose new demands for the selection, fabrication, and compatibility of the different battery components. Herein, solid polymer electrolytes (SPEs) based on poly(trimethylene carbonate) (PTMC) have been implemented in 3D-MB systems. 3D electrodes of two different architectures, LiFePO 4 -coated carbon foams and Cu 2 O-coated Cu nanopillars, have been coated with SPEs and used in Li cells. Functionalized PTMC with hydroxyl end groups was found to enable uniform and well-covering coatings on LiFePO 4 -coated carbon foams, which was difficult to achieve for nonfunctionalized polymers, but the cell cycling performance was limited. By employing a SPE prepared from a copolymer of TMC and caprolactone (CL), with higher ionic conductivity, Li cells composed of Cu 2 O-coated Cu nanopillars were constructed and tested both at ambient temperature and 60 °C. The footprint areal capacity of the cells was ca. 0.02 mAh cm -2 for an area gain factor (AF) of 2.5, and 0.2 mAh cm -2 for a relatively dense nanopillar-array (AF = 25) at a current density of 0.008 mA cm -2 under ambient temperature (22 ± 1 °C). These results provide new routes toward the realization of all-solid-state 3D-MBs.

Crystalline, three-dimensional (3D) structured lithium iron phosphate (LiFePO4) thin films with additional carbon are fabricated by a radio frequency (RF) magnetron-sputtering process in a single step. The 3D structured thin films are obtained at deposition temperatures of 600 °C and deposition times longer than 60 min by using a conventional sputtering setup. In contrast to glancing angle deposition (GLAD) techniques, no tilting of the substrate is required. Thin films are characterized by X-ray diffraction (XRD), Raman spectrospcopy, scanning electron microscopy (SEM), cyclic voltammetry (CV), and galvanostatic charging and discharging. The structured LiFePO4+C thin films consist of fibers that grow perpendicular to the substrate surface. The fibers have diameters up to 500 nm and crystallize in the desired olivine structure. The 3D structured thin films have superior electrochemical properties compared with dense two-dimensional (2D) LiFePO4 thin films and are, hence, very promising for application in 3D microbatteries.