New-Tech Europe Magazine | Jan 2018

array, made using lithography. Philippe Vereecken: “We are also working on an alternative for lithography so that the micropillar structures can be produced more cheaply. To do this, we will use process engineering based on foil, which will also make the battery flexible. In a later phase, we will further upscale the thin-film technology to greater energy and power densities so that it will be suitable for applications such as flexible, portable electronic devices.” The nano-composite electrolyte The electrolyte plays a key role in the development of batteries with larger storage capacities. As electrolytes gain higher ion conductivities, new battery architectures will become possible, with ever-thicker electrodes and hence higher storage capacity (more electrode material). The greater conductivity of the electrolyte means that the lithium ions will be able to span a longer dis-tance. With today’s solid-state electrolytes (such as LiPON, lithium-phosphate-salt doped with nitrogen) we can currently achieve intrinsic conductivities of only 10-7-10-6 S/cm, which means that the electrolyte layer is limited to a maxi- mum of one micrometer thick. To make batteries suitable for electric cars or home storage, we need conductivities of 10-3-10-2 S/cm. Philippe Vereecken: “At imec we invest quite some effort in the development of the solid electrolyte. We engineer materials at the nanoscale in order to achieve high ion conductivity. We use, for example, nanoporous silica, a materi-al that we have a great deal of experience with in the chip industry. When combined with a lithium salt into a composite, faster ion conduction can be achieved as the lithium ions move through this material along the surface of the silica. Their conductivity is further enhanced by what we call ‘pore confine- ment’ – the ‘locking’ of the lithium ions in the pore structure. As a result of

Fig 1: (Left) Schematic of the 3D thin-film battery; (right) Coated 3D micropillar structures. The pillars have a diameter and spacing of 2 micrometers.

Vereecken: “With a solid electrolyte, the electrodes can be placed closer together, making the battery more compact and thus delivering greater energy density. Unfortunately, the solid- state electrolytes available at the mo- ment have too low an ion conductivity. As a result, the lithium ions are only able to span a short distance so that these batteries only exist in the form of planar thin-film batteries. These batteries can only be used for micro-storage on account of their limited capacity. There are already a number of these bat-teries on the market today.” The 3D thin-film battery for micro-storage The solid-state design lends itself to completely new battery architectures, such as the compact 3D thin-film battery. Philippe Vereecken: “In this type of bat- tery, the thin-film stack is coated over a micro-structured substrate instead

of on a planar substrate. This method enables very thin films to be used for the electrode and the electrolyte, while achieving energy densities comparable with current-day battery technology. However, the actual capacity will still be limited due to the intrinsically small dimensions of these batteries. For this reason it is important that this battery can be recharged quickly or topped up in the back-ground so that the battery is never completely discharged and the microsystem is never without power. This is made possible by using thin films of at most a few hundred nanometers thick. As a result, the ions only need to travel a short distance and the battery can be recharged in a matter of minutes. These small form batteries are intended for microsystems such as implants, sensors and ‘smart cards’.” Imec has already demonstrated a proof- of-concept where the electrodes (LMO and LTO) were coated on a 3D micropillar

Fig 2: Schematic view of the solid-state powder-composite battery with nano-composite electrolyte.

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