One way to make Li-ion batteries more durable, safer, smaller and in particularly faster, is a transition towards all solid-state 3D thin-film Li-ion batteries.
By PHILIPPE VEREECKEN, principal scientist, imec, associate professor, KU Leuven
Applications like portable electronics, body area networks, wireless sensing networks and medical implants put severe pressure on energy storage technology development. As these devices become smarter and smaller at the same time, the demand for more powerful batteries with ever smaller volumes, larger storage capacity and higher lifetime grows. Of all known rechargeable systems, lithium ion (Li-ion) batteries provide the highest battery capacity and are therefore most popular for powering these devices. With a market share of more than 75%, they are currently the work horse of the rechargeable battery industry for portable applications. Besides portable electronics, Li-ion batteries are growing in popularity for large-scale storage solutions, like electric vehicle applications and temporary or local storage in future smart grids. Still, several material, structural and architectural innovations are needed to tune these batteries to the needs of future applications.
From liquid electrolytes to planar all solid- state batteries
Li-ion batteries belong to a class of rechargeable batteries, which means that the chemical conversion of the electrode material is reversible when an opposite cell voltage or current is applied. In a Li-ion battery, lithium ions move from the negative electrode during discharge and back when charging.
This reversible operation is enabled by using insertion electrodes, from which Li+ ions can be inserted or extracted. The most common positive electrode materials are currently lithium cobalt oxide (LCO), and lithium nickel cobalt manganese oxide (NMC). In the near future also lithium iron phosphate (LFP), lithium magnesium oxide (LMO) and lithium nickel cobalt aluminum oxide (NCA) will be used more and more. As negative electrodes, specialty graphite or Li4Ti5O12 are commonly used. For next generation batteries these might be complemented by silicon and silicon tin composites to increase the battery capacity.
The power characteristics of a battery cell strongly depend on the ionic conductance of the electrolyte which separates the electrodes. Current Li-ion battery technology makes use of liquid electrolyte solutions, consisting of lithium salts (such as LiPF6, LiBF4 or LiClO4) in an organic solvent (alkyl carbonates). They do have a very high conductivity of about 5-10S/cm at room temperature. In these battery types, however, the solid-electrolyte interface – which is formed as a result of the decom- position of the electrolyte at the negative electrode – limits the effective conductance (1-0.001S/cm2). Moreover, liquid electrolytes need expensive membranes to separate cathode and anode, and an impermeable casing to avoid leakage. This puts restrictions on the size and design of the batteries. And, since flammable and corrosive liquids are used, they suffer from safety and health issues.
Imec is looking towards solid-state Li-ion batteries, which are not only safer but allow scaling and even elimination of certain components. As such, they can be made with a higher effective energy and power density. In addition, they promise a longer lifetime and a broader temperature range of operation. These advantages encourage researchers to find innovative solutions for the main technological challenge: making a stable solid electrolyte component with high enough conductance for ions. An interesting approach is to scale down the thickness of the electrolyte. This way, an acceptable ionic conductance (e.g. 0.1-0.01S/cm2) can be obtained even for solid electrolytes with intrinsically low ion conductivity (e.g. 10-6S/cm). Scaling the electrolyte thickness is most efficiently done in a thin-film configuration and this presents some technical hurdles. Thin electrolyte films could lead to electrical shorts either through pinholes in the film or formation of conductive filaments (cf. a failure mechanism which is at the origin of resistive-RAM). Scaling down the film thickness also magnifies issues at the interfaces as in-diffusion regions may become considerable. This calls for advanced deposition techniques, like atomic layer deposition (ALD), to provide pinhole-free films and control the interfaces and as such the electrical properties of the battery stack.
From planar batteries to 3D thin-film batteries
FIGURE 1. Schematic of a planar (a) and 3D thin-film (b) battery with the following stack: current collector/ electrode/solid electrolyte/electrode/current collector.
A similar film scaling approach can be applied to the electrodes as well. Scaling down the electrode films significantly enhances the charging/discharging rate of the electrodes, and thus the battery power. But thinning down the films also diminishes the battery capacity, which depends directly on the total available amount of electrode material. In planar batteries, electrode films can therefore not be thinned below one micrometer (resulting in a maximum capacity of 0.07mAh/cm2 in case of a 1 μm thin LCO electrode). Fortunately, there is a way to compensate for the loss in electrode material and increase the battery capacity: the effective area can be increased by coating the thin-film stack on a micro- or nanostructured 3D structure (FIGURE 1).
A key enabling technology for these 3D thin-film batteries is micro- or nanostructuring of the substrate, for example by creating arrays of etched pillars in silicon. In order to maximize the battery power and capacity, an optimum condition in patterning density and film thickness need to be sought. Next, the thin films must be deposited onto the large surface areas in a conformal, pinhole-free and uniform way. Therefore, we need deposition techniques that allow a conformal coating of electrode and electrolyte materials onto high-aspect ratio 3D structured substrates. One of the options being explored at imec is a conformal stack of manganese and titanium based electrodes with a thin viscoelastic electrolyte interlayer (FIGURE 2). To achieve good conformality we rely on deposition techniques such as electrochemical deposition (ECD), chemical vapor deposition (CVD) and atomic layer deposition (ALD). As a first step, the thin-film batteries are fabricated on a Si platform taking advantage of the existing integrated circuit processing know-how at imec. These micro- batteries will be able to power microsystems such as wireless sensors. In a second stage, the processes and materials will be up-scaled to foil technology so that also batteries for portable electronics and eventually for local storage or electrical vehicles can be made. For this step, the know-how in the System in Foil program and the foil based process integration facilities in Holst Centre, our imec – TNO collaboration in Eindhoven are a big asset.
FIGURE 2. The electrode and electrolyte materials should be deposited onto microstructured surfaces
of the substrate (e.g., by the creation of silicon pillar structures, in order to maintain the battery capacity). This figure shows coating of a silicon pillar array with a 200nm film of electrochemically deposited MnO2 on a TiN diffusion layer.
A scaling technology roadmap
Many of the ideas for innovation in battery technology could well come from the IC industry, where the downscaling of the transistor has driven tremendous research efforts into new materials and nanotechnology. The driver behind the steady pace in scaling is a technology roadmap that sets out the specifications and material options for each new transistor generation. This evolution has led to the development of many new processes and techniques, such as advanced deposition techniques like CVD and ALD. Similar to the scaling of transistor technology (i.e. the transistor gate length and gate oxide), we believe a scaling technology roadmap for 3D thin-film batteries can gradually improve their performance (FIGURE 3).
FIGURE 3. Schematic showing the principle of scaling for 3D thin-film batteries.
The battery power (or charging and discharging rate) of the thin-film battery can be progressively improved by reducing the thickness of the electrode and electrolyte thin films (x axis in the figure). Each new generation (or node) would therefore require thinner pinhole-free, conformal, chemically uniform layers in ever higher aspect ratio features. Simultaneously, the pattern density of the 3D struc- tures must be increased to maintain the volume of the active electrode material (and hence the battery capacity). Or, more surface area can be provided without changing the pattern density, for example by using nanostructured pillars with increased aspect ratio (as indicated on the y axis of the figure). Alternatively, new materials with higher energy-density can be introduced for additional battery capacity.
As for CMOS scaling, the technological requirements for scaling 3D thin-film battery performance are demanding. We will need ever more advanced patterning, etch and deposition technologies to enable pinhole free, high quality coatings and to obtain conformality in extreme aspect ratios. Nevertheless, we believe that such a roadmap can lead us to ultrafast charging batteries. Such type of batteries could remove the need for more battery capacity for multimedia and computer devices as the battery can be constantly recharged in a wireless environment and thus never runs out. This novel mode of battery use is somewhat similar to that of autonomous microsystems where the battery is integrated with an energy harvester to recharge the battery when energy is available. Such solid-state batteries can indeed enable microsystems with full autonomous operation, needed for example in medical implants or automated sensor systems. The exceptional properties of solid-state batteries may also enable new technologies, like smart solar panels where storage is integrated on the backside of the solar panels, for example integrated in the roof of a car, bus or train.