Microbattery Applications Research Articles

Confining Iodine into Metal-Organic Framework Derived Metal-Nitrogen-Carbon for Long-Life Aqueous Zinc-Iodine Batteries.

Aqueous zinc-iodine batteries (AZIBs) are highly appealing for energy requirements owing to their safety, cost-effectiveness, and scalability. However, the inadequate redox kinetics and severe shuttling effect of polyiodide ions impede their commercial viability. Herein, several Zn-MOF-derived porous carbon materials are designed, and the further preparation of iron-doped porous carbon (Fe-N-C, M9) with varied Fe doping contents is optimized based on a facile self-assembly/carbonization approach. M9, with atomic Fe coordinated to nitrogen atoms, is employed as an efficient cathode host for AZIBs. Functional modifications of porous carbon hosts involving the doping species and levels are investigated. The adsorption tests, in situ Raman spectroscopy, and in situ UV-vis results demonstrate the adsorption capability and charge-discharge mechanism for the iodine species. Furthermore, experimental findings and theoretical analyses have proven that the redox conversion of iodine is enhanced through a physicochemical confinement effect. This study offers basic principles for the strategic design of single-atom dispersed carbon as an iodine host for high-performance AZIBs. Flexible soft-pack battery and wearable microbattery applications also have implications for future long-life aqueous battery designs.

Vertically Aligned Nanocomposite Thin Films Incorporating 3D-Architectures for Micro-Battery Applications

Vertically aligned nanocomposite (VAN) thin films have shown strong potential in oxide nanoionics but are yet to be explored in detail in solid-state battery systems [1,2]. VANs, typically grown by pulsed laser deposition (PLD), are chessboard-like structures comprised of pillars of one phase embedded in a matrix the second phase. They form by solid-state thermodynamic self-assembly, allowing the spontaneous formation of interdigitated battery components in a single deposition without additional processing steps such as etching. Their 3D architectures are attractive because they may allow enhancements in capacity, current and power densities [3]. Furthermore, VAN films grow with a preferential orientation, allowing for selective growth of the optimum crystallographic orientation for high performance by altering the substrate phase and/or orientation. Also, owing to their large interfacial surface areas, VAN could serve as models to study interfaces and solid-electrolyte interphase formation during cycling due to their high interfacial surface areas.In this poster, first we present highly crystalline and epitaxial vertically aligned nanocomposite PLD films comprised of a LixLa0.32±0.05(Nb0.7±0.1Ti0.32±0.05)O3±δ-Ti0.8±0.1Nb0.17±0.03O2±δ-anatase (LL(Nb,Ti)O-(Ti,Nb)O2) electrolyte/anode system, the first anode VAN battery system reported (Figure 1) [4]. This system exhibits an order of magnitude increased Li+ ionic conductivity over that in bulk Li3xLa1/3-xNbO3 (LLNO) and is comparable with the best available Li3xLa2/3-xTiO3 (LLTO) pulsed laser deposition films. Then, we will highlight the considerations and challenges that VANs pose, and how to overcome them [5]. Finally, we will also present our recently submitted work on VAN based cathodes [6]. Our carefully selected materials combination exhibits remarkable high-rate performance, achieving discharge capacities > 70 mAh g-1 up to ~200 C with clear redox behavior of the cathode-of-choice. The enhanced electrochemical performance is enabled by the 3D VAN architecture. Interestingly, we elucidate the relationship between pillar morphology and electrochemical performance, namely there is an optimum size and orientation of cathodic nanopillars such that high discharge capacity under high-rate regimes is possible. These works open up the possibility of incorporating VAN films into an all solid-state battery, either as electrodes or electrolytes, by the pairing of suitable materials.

Pulsed laser deposited V2O3 thin-films on graphene/aluminum foil for micro-battery applications

The development of new thin-film cathodes triggered a recent research interest in energy storage applications. Over the past years, vanadium oxides have been extensively explored as promising electrodes for batteries owing to their rich valence states and remarkable electrochemical properties. Herein, we report on the synthesis of undoped and Sn doped V2O3 thin-films on graphene (G)/Al foil by pulsed laser deposition followed by rapid thermal annealing in N2 at low temperature (∼430 °C). The obtaining V2O3 phase on graphene/Al foil (G/Al) has been confirmed by X-ray diffraction and Raman and X-ray photoelectron spectroscopy analyses. The synthesized vanadium oxide films were tested as cathodes in coin cells. The electrochemical properties have been systematically investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge discharge (GCD) measurements. A superior electrochemical performance was observed for the V2O3 on G/Al structures, with an initial capacity of around 300 mAh g−1, with respect to the bare G/Al electrode. The use of the Sn-doped (5 mol%) V2O3 thin-films improved slightly the initial capacity up to a value of ca. 311 mAh g−1. Both V2O3/G/Al and Sn-doped V2O3/G/Al exhibited excellent cycling performances after 40 cycles with a capacity maintenance at a C-rate C/20 of 317 mAh g−1. Long-term cycling test (up to 200 cycles) showed that the Sn doping could be an excellent strategy to improve the stability of the electrodes, which yielded a capacity loss of only 0.128% per cycle. Possible mechanisms are presented and discussed. This work could serve as point of reference for future developments in the field of batteries employing vanadium oxide-based thin-films deposited by physical vapor deposition techniques.

Superior lithium storage in Fe2O3 nanoporous arrays endowed by surface phosphorylation and bulk phosphorous doping

Fe2O3 film anodes are promising for application in lithium-ion micro-batteries in terms of their high theoretical capacity, earth-abundancy, and being environmentally friendly. Nevertheless, the low intrinsic electronic conductivity, sluggish Li+ diffusion, and unstable electrode–electrolyte interface stemming from cycling within Fe2O3 film limit its cyclic stability and rate capability. Herein, extraordinary cyclability and rate performance boosting of nanoporous Fe2O3 film anode are achieved using facile phosphorous heteroatom doping. The unique P-Fe2O3 nanoporous film can reversibly exhibit a capacity of 725 mAh/g at 248 mA g−1 for 300 cycles with a capacity retention of 94.5 %, which is approximately 15 times higher than that without P doping. Moreover, the P-Fe2O3 nanoporous film can retain a capacity of 401 mAh/g at a high rate of 2.48 A/g and the initial coulombic efficiency can reach 90.4 %. Experimental results and density functional theory analysis verify that the enhanced performance derives from the synergistic effect of surface phosphorylation layer and bulk P doping defect, which can promote electron and Li+ transportation meanwhile enhancing surface faradaic redox sites. The insight from this work can provide a new avenue for the rational design of other advanced film electrodes through defect and surface engineering simultaneously.

In Situ Liquid Electrochemical TEM Investigation of LiMn1.5 Ni0.5 O4 Thin Film Cathode for Micro-Battery Applications.

Micro-batteries are attractive miniaturized energy devices for new Internet of Things applications, but the lack of understanding of their degradation process during cycling hinders improving their performance. Here focused ion beam (FIB)-lamella from LiMn1.5 Ni0.5 O4 (LMNO) thin-film cathode is in situ cycled in a liquid electrolyte inside an electrochemical transmission electron microscope (TEM) holder to analyze structural and morphology changes upon (de)lithiation processes. A high-quality electrical connection between the platinum (Pt) current collector of FIB-lamella and the microchip's Pt working electrode is established, as confirmed by local two-probe conductivity measurements. In situ cyclic voltammetry (CV) experiments show two redox activities at 4.41 and 4.58/4.54 V corresponding to the Ni2+/3+ and Ni3+/4+ couples, respectively. (S)TEM investigations of the cycled thin-film reveal formation of voids and cracks, loss of contact with current collector, and presence of organic decomposition products. The 4D STEM ASTAR technique highlights the emergence of an amorphization process and a decrease in average grain size from 20 to 10 nm in the in situ cycled electrode. The present findings, obtained for the first time through the liquid electrochemical TEM study, provide several insights explaining the capacity fade of the LMNO thin-film cathode typically observed upon cycling in a conventional liquid electrolyte.

High performance LATP thin film electrolytes for all-solid-state microbattery applications

A NASICON solid electrolyte Li1+xAlxTi2−x(PO4)3 (LATP) in a thin film form has been developed with ionic conductivities as high as 0.1 mS cm−1. A route to achieve a 100-fold increase in the conductivity through post-annealing is discussed.

LiCoO2 thin film cathode sputtered onto 500 °C substrate

LiCoO2 thin film cathode, which has attracted considerable attention in the past decades due to its promising application in micro-batteries, is commonly produced by magnetron sputtering combined with post-annealing treatment. However, the high temperature treatment hinders its compatibility toward a majority of substrates, while the moderate temperature post-annealing constrains its crystallinity. In this study, LiCoO2 film cathode is sputtered onto the 500 °C substrate. The in-situ annealed LiCoO2 thin film cathode is highly crystallized as hexagonal structure without cubic impurities, exhibiting a capacity retention of 75% at 1 C after 200 cycles and excellent rate performance with 55% of initial capacity at 70 C in liquid electrolyte-based cell. All solid-state thin film Li battery exhibits excellent cycle stability. The excellent electrochemical performance is ascribed to highly crystallized LiCoO2 thin film, which is trigged by high kinetic energy of the sputtered species. On the contrary, the LiCoO2 film fabricated by post-annealing at 500 °C shows much poorer electrochemical performance due to its lower crystallinity, indicating that the annealing mode is crucial for the sputtering. This work provides a new option to deposite high quality LiCoO2 thin film via in situ annealing at a relatively low temperature.

Understanding the effect of p-, n-type dopants and vinyl carbonate electrolyte additive on electrochemical performance of Si thin film anodes for lithium-ion battery

Amorphous silicon (Si) is one of the most prospective high performance anode material for lithium-ion batteries and thin films of this material are promising for microbattery applications. Undoped, p- and n-type doped Si thin films were deposited by madgnetron sputtering on a three-dimensional current collector designed to accommodate the volume expansion of anode upon charge-discharge cycling, improve the active material adhesion, and to enhance the rate capability of the anode. The designed Si thin film anodes with and without doping were first investigated in lithium cell using various electrochemical techniques confirming the positive effect of doping on their performance. Raman spectroscopy was performed before and after cycling the electrodes to further elucidate such effects. The trends in change of an electrolyte during first lithiation/delithiation were observed by in situ Raman technique. Along with this, the morphology features of a solid electrolyte interphase (SEI) layer formed on the surface of the electrode and its modifications upon addition of 5% vinyl carbonate (VC) to the electrolyte were studied. The n-type doping and unique SEI layer formed with poly(VC) polymeric species suppressed the electrode crack formation preventing the capacity loss due to the material disintegration upon cycling, which resulted in an outstanding cycling performance of Si thin film with the capacity of 1305 mAh g−1 retained over 400 cycles.

Powering the Internet of Things

Internet connected devices are limited by the performance of microbatteries that power them. In this issue of Joule, Hur et al. develop microbatteries with tall 3D electrodes that have high areal energy densities. These microbatteries were fabricated on a silicon wafer using silicon and polymer processing that are compatible with microelectronics, paving the way for future microbattery and microelectronics integration.

Effects of Cell Geometry on Rate Performance of Secondary Batteries

3D geometries for microbattery applications have been explored to increase capacity per footprint area, relevant for small footprint area micro devices [1] [2]. Simulations of interdigitated Li-ion batteries suggest 4 orders of magnitude increase in areal capacity at 330 C-rate from 0.58 Ah/m2in the conventional geometry to 318 Ah/m2 for the 3D geometry [3]. Realization of practical 3D battery designs can potentially decouple their power and energy [4], properties that are historically coupled in conventional battery design, which might have significant implications for high power devices. Using commonly found commercially available materials and tools, we designed and conducted a simple experiment to elucidate how battery performance can be improved by varying the relative arrangements of the active and inactive components. Examples include using Li/LiCoO2 2032-type coin cells with 1M LiPF6 and carbonate containing electrolytes. The maximum active ion diffusion length required to fully delithiate the LiCoO2 electrodes during cycling was increased by 400-6500 μm compared with standard reference cells, and the effect on rate performance was measured between C/24 and C/3. Results suggest that extending the maximum active ion diffusion path length by >1600 μm still allows for attaining specific capacity that matches that of the reference cell at C/24. We discuss possible new geometries and optimized designs in the framework of battery performance. [1] C. Liu, E.I. Gillette, X. Chen, A.J. Pearse, A.C. Kozen, M.A. Schroeder, K.E. Gregorczyk, S.B. Lee, G.W. Rubloff. Nature Nanotechnology 9 (2014) 1031-1039. [2] T.S. Arthur, D.J. Bates, N. Cirigliano, D.C. Johnson, P. Malati, J.M. Mosby, E. Perre, M.T. Rawls, A.L. Prieto, B.Dunn. MRS Bulletin 36 (2011) 523-531. [3] D. Miranda, C.M. Costa, A.M. Almeida, S. Lanceros-Méndez. Applied Energy 165 (2016) 318-328. [4] N. Cirigliano, G. Sun, D. Membreno, P. Malati, C.J. Kim, B. Dunn. Energy Technology 2 (2014) 362-369.

Free-Standing Oxide Nanotube Array Electrodes for High Energy Density and Power Density Li-Ion Batteries

During the past few years, with the increasing power demand generated by applications from portable devices to electric vehicles, more and more emphasis is put on manufacturing high energy and power density Li-ion batteries, i.e., to maximize the capacities while retaining a high rate capability. So far, the studies have been mainly dedicated to the development of powder type electrode materials and relatively little attention has been paid to studies of other electrode architectures. While composite electrodes containing a mixture of the active material powders, binders and conductive additives still are commonly used, such electrodes often yield poor material utilization, undefined material/component arrangements and a lot of complex interfaces. In the present work, we demonstrate that various highly ordered, free-standing oxide nanotube array electrodes, fabricated by electrochemical anodization approaches, can be used either for high energy density and power density Li-ion microbattery applications or as model monolithic electrodes for electrode engineering studies. By using anatase TiO2 nanotube electrodes (as an example of an intercalation type material), an areal capacity of 0.37 mAh cm-2 (i.e., 40 mAh g-1) at a rate of 10C (using a (dis-)charge current density of 9 mA cm-2), and 1 mAh cm-2 (i.e., 91 mAh g-1) at a rate of C/5), can be achieved. [1] Well-defined monolithic anatase TiO2 nanotube electrodes with fine-tuned nanotube size gradients (including tube length, diameter and wall thickness) can also be manufactured using a bipolar electrochemistry approach. [2] The gradient nanotube electrodes can provide excellent rate performance, with capacities from of 0.16 mAh cm-2 or 169 mAh g-1 at a rate of C/5 to 0.04 mAh cm-2 or 42 mAh g-1 at a rate of 50C. In addition, free-standing Nb2O5 nanotube electrodes, which can be cycled for 10000 cycles with only a 20% loss of initial capacities, can provide unprecedented high-rate battery performances, i.e. the capacities from of 0.1 mAh cm-2 or 110 mAh g-1 at a rate of C/5 to 0.04 mAh cm-2 or 44 mAh g-1at a rate of 100C.[3] Some recent work carried out to investigate highly ordered Fe3O4 nanotubular/nanoporous electrodes as a prototype free-standing conversion electrode will also be described.

Mixed Valence Niobium Oxides for High Power 3D Microbattery Applications

Research in microbatteries is stimulated by the need for miniature power sources for use in autonomous sensing, wireless communications, medical implants and other microscale devices (1-10 mm3).1, 2 In such applications wherein physical space is at premium, sufficiently small, yet powerful, energy storage devices are desired. Currently, either thin or thick film microbatteries are in common use; however, 2D geometries suffer from severe limitations in that they are unable to deliver both high power and high energy densities simultaneously. The requirement for high energy and power densities can be met using (1) high energy materials that are structurally and electronically suited for fast rate cycling (examples: b-TiO2, Nb2O5, α-MoO3…)3-5 and (2) 3D–structured electrodes which allow for the use of an increased amount of active material without affecting the power delivery.1, 2, 6-8 In the latter context, a variety of electrode architectures have been considered including various types of carbon foams.9-11 By virtue of its bicontinuous architecture and reasonably high electrical conductivity, a carbon foam electrode provides an effective conduit for the transport of electrons and an interconnected porosity for the percolation of the electrolyte throughout the electrode. As a result, an intimate contact can be maintained amongst the current collector (the carbon foam), the active material (deposited onto the carbon foam) and the electrolyte (flooding the voids) over a microscale length. This in turn ensures efficient utilization of the active material at the desired cycling rates. It should be noted, however, that practical applications require optimal porosity and void sizes to ensure an acceptable trade-off between power and energy densities. To this end, part of the research in microbatteries focuses on the development of suitable porous electrodes. This presentation highlights the fabrication of 3D microstructured electrodes using emulsion-templated carbon foams as scaffold for niobium oxide nanoparticles. As compared to commercially available carbon foams, they have higher specific surface areas and void sizes that are two orders of magnitude smaller.10-12The void size distribution, surface areas and electrical conductivity of the carbon foams can be fine-tuned by optimizing the synthesis conditions. A brief discussion will be presented in this regard. Niobium is known to form both stoichiometric and non-stoichiometric oxides. The mixed valency present in the nonstoichiometric oxides is associated with increased electrical conductivity.13 The semiconducting nature of these oxides makes them ideal candidates for high power applications. In this particular case, the carbon foam has been used for dual purposes: (1) to entrap the oxide nanoparticles and (b) to promote the reduction of part of the Nb5+ to Nb4+ at elevated temperatures: Nb2O5 + δC à NbO2.5-δ + δCO Two types of mixed valence oxides have been synthesized and investigated for potential applications in high power 3D microbatteries. Detailed discussions on the structures and electrochemical behaviors of the oxides will be offered in the presentation. Figure 1. (A) SEM of NbOx-coated carbon foam (B) CV of electrode in (A) 1. K. Edström, D. Brandell, T. Gustafsson and L. Nyholm, Electrochem. Soc. Interface, 2011, 20, 41-46. 2. J. W. Long, B. Dunn, D. R. Rolison and H. S. White, Chem. Rev., 2004, 104, 4463-4492. 3. V. Augustyn, J. Come, M. A. Lowe, J. W. Kim, P.-L. Taberna, S. H. Tolbert, H. D. Abruña, P. Simon and B. Dunn, Nat Mater, 2013, 12, 518-522. 4. M. Zukalová, M. Kalbáč, L. Kavan, I. Exnar and M. Graetzel, Chem. Mater., 2005, 17, 1248-1255. 5. T. Brezesinski, J. Wang, S. H. Tolbert and B. Dunn, Nat Mater, 2010, 9, 146-151. 6. M. Valvo, M. Roberts, G. Oltean, B. Sun, D. Rehnlund, D. Brandell, L. Nyholm, T. Gustafsson and k. Edstrom, Journal of Materials Chemistry A, 2013. 7. M. Roberts, P. Johns, J. Owen, D. Brandell, K. Edstrom, G. El Enany, C. Guery, D. Golodnitsky, M. Lacey, C. Lecoeur, H. Mazor, E. Peled, E. Perre, M. M. Shaijumon, P. Simon and P.-L. Taberna, J. Mater. Chem., 2011, 21, 9876-9890. 8. H. P. James, Z. Hui Gang, C. Jiung, V. B. Paul and P. K. William, Nat. Commun., 2013, 4, 1732-1732. 9. P. Johns, M. Roberts and J. Owen, J. Mater. Chem., 2011, 21, 10153-10159. 10. H. D. Asfaw, M. Roberts, R. Younesi and K. Edstrom, Journal of Materials Chemistry A, 2013, 1, 13750-13758. 11. H. D. Asfaw, M. R. Roberts, C.-W. Tai, R. Younesi, M. Valvo, L. Nyholm and K. Edstrom, Nanoscale, 2014, 6, 8804-8813. 12. D. Wang, N. L. Smith and P. M. Budd, Polym. Int., 2005, 54, 297-303. 13. O. G. D’yachenko, S. Y. Istomin, A. M. Abakumov and E. V. Antipov, Inorg. Mater., 36, 247-259. Figure 1

High-performance flexible nanoporous Si-carbon nanotube paper anodes for micro-battery applications

Nanoporous Si has been grown by pulsed laser deposition on a free-standing carbon nanotube (CNT) paper sheet for micro-battery anodes. The Si deposition shows conformal coverage on the CNT paper, and the Si-CNT paper anodes demonstrate high areal capacity of ∼1000 μAh cm−2 at a current density of 54 μA cm−2, while 69% of its initial capacity is preserved when the current density is increased by a factor 10. Excellent stability without capacity decay up to 1000 cycles at a current density of 1080 μA cm−2 is also demonstrated. After bending along the diameter of the circular paper disc many times, the Si-CNT paper anodes preserve the same morphology and show promising electrochemical performance, indicating that nanoporous Si-CNT paper anodes can find application for flexible micro-batteries.

Three-dimensional interconnected Nicore–NiOshell nanowire networks for lithium microbattery architectures

3D interconnected Nicore–NiOshell electrodes are simple to realize and scalable and provide superior lithium storage properties. Could this be the ideal configuration for microbattery applications?

Three-Dimensional, Fibrous Lithium Iron Phosphate Structures Deposited by Magnetron Sputtering.

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.

A polymerized C60 coating enhancing interfacial stability at three-dimensional LiCoO2 in high-potential regime

The interfacial instabilities, including side reactions due to electrolyte decompositions and Cobalt (Co) dissolutions, are the main detrimental processes at LiCoO2 cathode when a high-voltage window (>4.2 V) is applied. Nevertheless, cycling the cathode with a voltage above 4.2 V would deliver an increased gravimetric capacity, which is desired for high power battery operation. To address these drawbacks, we demonstrate a synergistic approach by manufacturing the three-dimensional high-temperature LiCoO2 electrodes (3D HT-LCO) using laser-microstructuring, laser-annealing and subsequent coating with polymerized C60 thin films (C60@3D HT-LCO) by plasma-assisted thermal evaporation. The C60@3D HT-LCO cathode delivers higher initial discharge capacity compared to its theoretical value, i.e. 175 mA h g−1 at 0.1 C with cut-off voltage of 3.0–4.5 V. This cathode combines the advantages of the 3D electrode architecture and an advanced C60 coating/passivation concept leading to an improved electrochemical performance, due to an increased active surface area, a decreased charge transfer resistance, a prevented Co dissolution into the electrolyte and a suppressed side reaction and electrolyte decomposition. This work provides a novel solution for other cathode materials having similar concerns in high potential regimes for application in lithium-ion microbatteries.

Conductivity Enhancement and Thin-Film Processing of Li4Ti5O12(LTO) Spinel for Li Battery Applications

Lithium titanium oxide spinel Li4Ti5O12 has been reported to be a zero-strain lithium insertion host material because it exhibits extremely small variations of the lattice parameters during the charge and discharge processes. In general, Poor adhesion between thin-film electrode and substrates is the common cause for performance degradation of thin-film electrode during repeated charge/discharge tests. Thus, Li4Ti5O12 spinel may be the ideal candidate for the anode of thin-film Li ion battery for long cycle applications. However, unmodified Li4Ti5O12 usually exhibits poor rate performance, which mainly results from its low electronic conductivity (ca. 10-13 S·cm-1) and moderate Li+ diffusion coefficient (10-8cm2·s-1). In addition, different electrochemical behaviors will be expected when Li4Ti5O12 spinel is prepared in thin-film compared to that in power forms. Therefore, the objective of this work is (1) to enhance the conductivity of Li4Ti5O12 spinel by substitution of aliovalent dopants for Ti ions, (2) to fabricate thin film of Li4Ti5O12spinel by using RF-sputtering technique, (2) to investigate the effect of annealing temperature/atmospheres on the electrical/electrochemical properties of thin film Li4Ti5O12, (3) to evaluate the feasibility of using thin film of Li4Ti5O12 as anode for Li microbattery applications. In this study, the Li4Ti5O12 thin film was deposited on either the Si substrate or glass substrate by sputtering technique. The crystallization of deposited Li4Ti5O12 thin film will be enhanced by annealing at various atmospheres and temperatures. The single phase Li4Ti5O12 was finally obtained after co-sputtering of Li-containg and Ti-containing targets. The crystal structure and crystallization will be examined by glancing XRD. The resistivity of Li4Ti5O12 thin film will be evaluated by four probe technique and our novel micromeaurement system. The capacity of Li4Ti5O12 thin film will be evaluated from charge/discharge tests.

LiMn2O4 nanorod arrays: A potential three-dimensional cathode for lithium-ion microbatteries

Although three-dimensional (3D) microbatteries represent great advantage compared to their two-dimensional counterparts, the fabrication of 3D cathode is still a challenge, which holds back the further development of 3D microbatteries. In this work, we present a novel approach for fabrication of LiMn2O4 nanorod arrays as 3D cathode for microbatteries. α-MnO2 nanotube arrays are firstly grown on the Pt substrate as the template, and LiMn2O4 nanorod arrays are then prepared by lithiation of α-MnO2 nanotube arrays in molten salt followed by 800°C annealing in air. In the half cell test, the 3D LiMn2O4 nanorod arrays exhibit both high gravimetric capacity (∼130mAhg−1) and areal capacity (∼0.25mAhcm−2), while maintaining good cycling stability and rate capability. The facile synthesis and superior electrochemical performance of the three-dimensional LiMn2O4 cathode make it promising for application in microbatteries.

Aligned TiO2 nanotube/nanoparticle heterostructures with enhanced electrochemical performance as three-dimensional anode for lithium-ion microbatteries

A novel TiO2 three-dimensional (3D) anode with an aligned TiO2 nanotube/nanoparticle heterostructure (TiO2 NTs/NPs) is developed by simply immersing as-anodized TiO2 NTs into water and further crystallizing the TiO2 NTs by post-annealing. The heterostructure, with its core in a tubular morphology and with both the outer and inner surface consisting of nanoparticles, is confirmed by FESEM and TEM. A reversible areal capacity of 0.126 mAh · cm−2 is retained after 50 cycles for the TiO2 NTs/NPs heterostructure electrode, which is higher than that of the TiO2 NTs electrode (0.102 mAh · cm−2 after 50 cycles). At the current densities of 0.02, 0.04, 0.06, 0.08, 0.10 and 0.20 mA · cm−2, the areal capacities are 0.142, 0.127, 0.117, 0.110, 0.104 and 0.089 mAh · cm−2, respectively, for the TiO2 NTs/NPs heterostructure electrode compared to the areal capacities of 0.123, 0.112, 0.105, 0.101, 0.094 and 0.083 mAh · cm−2, respectively, for the the TiO2 NTs electrode. The enhanced electrochemical performance is attributed to the unique microstructure of the TiO2 NTs/NPs heterostructure electrode with the TiO2 NT core used as a straight pathway for electronic transport and with TiO2 NP offering enhanced surface areas for facile Li+ insertion/extraction. The results described here inspire a facile approach to fabricate a 3D anode with an enhanced electrochemical performance for lithium-ion microbattery applications.

Facile synthesis of chain-like LiCoO2 nanowire arrays as three-dimensional cathode for microbatteries

Three-dimensional microbatteries have emerged as a new direction for powering microelectronic devices, where the three-dimensional nanostructured electrode is the key component for microbatteries to achieve high power density and high energy density in a small footprint. In this work, we present a novel approach for fabrication of LiCoO2 nanowire arrays as three-dimensional cathode for microbatteries. Mesoporous low-temperature LiCoO2 nanowire arrays can be directly prepared by a two-step hydrothermal method and they can be easily converted into chain-like high-temperature LiCoO2 nanowire arrays through further calcination. The layered LiCoO2 nanowire arrays exhibit both high gravimetric capacity and areal capacity, while maintaining good cycling stability and rate capability. The facile synthesis and superior electrochemical performance of the three-dimensional LiCoO2 cathode make it promising for application in microbatteries. Hui Xia from Nanjing University of Science and Technology in China and colleagues report a three-dimensional (3D) nanowire battery cathode that packs high power and energy into a tiny framework. The team turned lithium cobalt oxide - a compound widely used for lithium-ion battery cathodes - into freestanding, 3D nanowire arrays on metal substrates using a simple two-step hydrothermal synthesis technique. Further heat treatment transformed the nanowires into chain-like structures with first-rate electrochemical and recharging properties thanks to the material's short ion transport lengths and large surface area for energy storage. The cathode helps solve problems associated with delivering sufficient power and high storage capacities for on-chip battery applications - a finding that can aid in the design of advanced microscale devices such as drug delivery systems and wireless sensors. Mesoporous low-temperature LiCoO2 nanowire arrays can be directly prepared by a two-step hydrothermal method and they can be easily converted into chain-like high-temperature LiCoO2 nanowire arrays through further calcination. The layered LiCoO2 nanowire arrays exhibit both high gravimetric capacity and areal capacity, while maintaining good cycling stability and rate capability, make them promising for application in microbatteries.