3D Structuring of Thin Film Solid State Li-Ion Cells for Fast Charging Micro-Batteries
Authors: Vereecken Philippe M. 
Published: February 2020

High energy and power density are the main requirements of today’s high demanding applications in consumer electronics. Lithium ion batteries (LIB) have the highest energy density of all known systems and are thus the best choice for rechargeable micro-batteries. Liquid electrolyte LIBs present limitations in safety, size and design, where all-solid state thin film batteries are considered to overcome these restrictions for small devices. Although planar all-solid state thin film LIBs are at present commercially available, they provide low capacity (<1mAh/cm2) which limits their application scenario. By using micro-structured surfaces (i.e. 3D batteries) and appropriate conformal coating technologies (i.e. electrochemical deposition, ALD), the capacity can be increased while still keeping a high rate performance. The main challenges in the introduction of solid-state LIBs are low ionic conductance and limited cycle life time due to mechanical stress and shearing interfaces. Novel materials and innovative nanostructures have to be explored in order to overcome these limitations. Thin film 3D compatible materials need to provide with the necessary requirements for functional and viable thin-film stacks. Thin film electrodes offer shorter Li-diffusion paths which allow them to be used at ultra-fast charging rates while keeping their maximum capacities. Thin film electrolytes with intrinsically high ion conductivity (~10-3 S/cm) do exist, but are not electrochemically stable. On the other hand, electronically insulating electrolytes with a large electrochemical window and good chemical stability are known, but typically have intrinsically low ionic conductivities (<10-6S/cm). In addition, there is the need for conformal deposition techniques which can offer pinhole-free coverage over large surface areas with large aspect ratio features for electrode, electrolyte and buffer layers.
In this paper, we present the fabrication of 3D thin-film batteries using high aspect ratio silicon pillar arrays providing area enhancement of over 25x. The pillars are conformally coated with LiMn2O4 (LMO) and LixTiO2 as positive and negative electrodes, respectively. For the electrolyte, thin-film composite electrolyte films are being developed. Where we currently focus on 60 μm high pillars with a 2 μm diameter and pitch, the energy and power output per unit area of the battery can be increased by scaling towards higher aspect ratio pillars. Indeed, by increasing the height of the pillars, more active material per unit area can be deposited, resulting in a higher capacity. Simultaneously, the power output of the battery improves due to the increased internal surface of the 3D battery. Notice that a similar approach is not possible with conventional thin-film batteries. Increasing the active film thickness in these batteries results in more capacity, but this goes at the cost of power output due to the increased resistance of the thicker active layers. In addition, by decreasing the pitch between the pillars, the film thickness can be scaled down. We show that the available capacity of both the manganate and titania based films can be increased by scaling down the film thickness below 100 nm without affecting cycle life time. The thinnest films show the highest volumetric capacity and the best cycling stability. The increased stability of the LMO films below 50 nm allows cycling in both the 4 and 3V potential region, resulting in a high volumetric capacity of 1.2Ah/cm3. The potential of 3D thin-film batteries with respect to attainable energy density and C-rate performance will be discussed.

Figure 1