Microbatteries

Further, tungsten oxysulfide films, WOyS, have shown promising behavior as positive electrodes in microbatteries, unlike WS2 that is not suitable as cathode in lithium cells. Using amorphous thin films of WO1.05S2 and WO1.35S2.2 in the cell Li/LiAsFe, 1 M ethyl-methyl sulfone (EMS)/W03,Sz, Martin-Litas et al. [80] obtained current densities up to 37 xA cm between 1.6 and 3 V. In these cathode materials, 0.6 and 0.8 lithium per formula unit, respectively, could be intercalated and de-intercalated reversibly. 

Martin-Litas 1, Vinatier P, Levasseur A, Dupin JC, Gonbeau D (2001) Promising thin films (WOi 05S2 and WO1.35S2.2) as positive electrode materials in microbatteries. J Power Sources 97-98 545-547... 

Other developments in the area of solid state lithium batteries include prototype production and testing of thin-film microbatteries at Oak Ridge National Laboratory in the USA. The fabrication involves electrode and electrolyte film deposition to form compact layers of thickness of the order of few microns. The cell uses a lithium anode, an amorphous Li3 3PO3.9N0.17 solid electrolyte and an amorphous V205 cathode ... 

J.R. Akridge and M. Balkanski, Solid State Microbatteries, NATP ASI Series, Plenum Press, New York, 1990. 

Materials obtained by pyrolysis of pitch-polysilane blends have been extensively studied as carbon materials containing Si [157-161], For some of these materials, ca. 600mAh/g of Crev for Li insertion, as well as small irreversible capacities and small hysteresis effects, were reported. It has been shown that the materials contain nanodispersion of Si-O-C and Si-O-S-C instead of nanodispersed Si particles [162-165], Furthermore, the oxygen and sulfur contents are proved to be correlated to the irreversible capacity. There is a report about the fabrication of porous Si negative electrodes with 1-D channels, where the usefulness of the fabricated negative electrodes for rechargeable microbatteries is also suggested [166],... 

Microbatteries which are small devices for energy storage. 

Humble, P.H. Harb, J.N. Optimization of nickel-zinc microbatteries for hybrid powered microsensor systems. J. Electrochem. Soc. 2003, 150, A1182-A1187. 

Sensors, CMOS memories, credit cards, implantable medical devices, MEMS 200 Microbatteries... 

Naturally, such conditions require different criteria for the thin-film microbatteries cell design compared to the larger batteries systems. This section will briefly review the achievements reported on the design and fabrication of Li microbatteries. 

Fig. 2.1 Schematic representation of the construction of a solid-state microbattery. Reprinted from [4], with permission from Elsevier...

Thin film deposition technologies are widely used for the fabrication of microbatteries. They can be divided into two main categories Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). 

Flash evaporation is another techniqne for the deposition of films whose constituents have different vapor pressures. Small quantities of the constituents in the desired ratio are continuously dropped at a predetermined rate from a vibration feeder into a sufficiently heated crucible or boat so that they are evaporated instantly. The temperature must be sufficiently high to evaporate the less volatile material. Figure 2.3 shows the flash-evaporation apparatus used to grow the different layers that compose a microbattery metallic contacts, cathode, electrolyte, lithium anode deposited under vacuum of 10-100 mPa pressure. This system has two vacuum chambers. Vacuum chamber A is devoted to evaporation of In-Se films while lithium and glass films are formed in chamber B. Two interlock mechanisms are used the first one transfers the grown In-Se film to the chamber B and the second interlock system is ntilized to carry lithium pieces from an inert-gas glove box to the evaporation boat. 

Fig. 2.3 Schematic representation of a flash evaporation system used to fabricate microbatteries. Reprinted from [3], Copyright Elsevier...

For power sources with reduced size, increased flexibility, longer lifetime, and increased reliability, there are also some approaches concerning the hybridization of the Li microbatteries with energy conversion devices (i.e. solar cells and piezo-electric generator) [11, 12], This is claimed to be particularly feasible for autonomous sensors that do not have a direct connection to a host for either power or communication. 

This photoresist is patterned by photolithography and pyrolyzed in an inert environment at high temperature to form the carbon electrode [27-30], In a more recent work, laser excitation has been used to both pyrolyze the film and to write the electrode pattern. [31], Figure 2.9 shows an example of the 3-D microbattery that has been actually fabricated using the C-MEMS process [32]. 

Fig. 2.10 Design of C-MEMS 3-D microbattery and a typical SEM of low aspect ratio C-MEMS battery arrays. Both the electrodes and contact fingers are made of carbon. Reproduced from [32] with permission from ECS - The Electrochemical Society...

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