New Materials and Processes for High Energy Density Solid State Batteries
Authors: Hayden Brian Elliot , Lee Christopher Edward , Laughman David , Noble Robert , Turner Louise , Clarke Owain , Casey Patrick , Jazdanian Andreas 
Published: February 2020

Thin film all-solid-state lithium ion batteries have significant advantages for distributed wireless sensor network applications: Fit and forget batteries with fast charge and discharge rates, high energy density and low leakage currents make them compatible with combinations of energy harvesters, microprocessors, communications devices and sensors. While the energy budget of these systems continually reduces, the incumbent solid-state chemistries and production methodologies mean that available energy and power on a small footprint from such batteries remains insufficient. 

Combinatorial synthesis of thin films using proprietary evaporation methods [1], and high throughput (HT) characterisation and screening, has enabled the optimisation of the component materials, and the combination of these materials, in solid-state batteries. This approach has been validated using benchmark materials, for example, solid-state electrolytes (e.g., Li3xLa2/3-xTi1/3-2xO3, LLTO) [2]. The advantages of this approach, particularly with regard to lithium containing materials, was more recently [3] applied to cathode / electrolyte interface modification, and here will be exemplified for cathode, anode and conventional and new solid electrolyte materials. In particular, the solid electrolyte LiBSiO has also been optimised using HT-PVD: deposited at room temperature, the materials family is amorphous and yields ionic conductivities of ca. 2 x 10-6 S.cm-1. 

The co-evaporation synthetic method for combinatorial synthesis and materials and device optimisation also has considerable advantage in the manufacture of the solid-state batteries themselves. The key advantages are that the component active materials and their phases can be synthesised in-situ in a single step at temperatures considerably lower than those required when using other deposition methods. Compositional control, oriented and dense crystalline materials can be accessed at excellent rates, and new thin film amorphous anodes and solid-state electrolyte materials become accessible. These aspects will be exemplified through solid-state batteries manufactured with LiCoO2 cathodes synthesised at ca. 400 °C without post annealing / processing. The cathode material has a dense microstructure and retains a crystalline orientation along the (003) axis well beyond a thickness of 1 µm. 

The performance of a 275 µAh solid-state battery on a 1 cm2 footprint, which has been optimised using this approach and manufacturing method will be presented. Batteries are produced in batch mode on an automated cluster tool on 150 mm substrates. The effective utilisation of the cathode in the overall battery is ca. 60 µAh/cm2/µm, and the effective performance under various charge and discharge rates will be presented, together with evidence of good stability with cycling. The cycling data shown below are for smaller prototype batteries manufactured on a 150 mm silicon wafer. The significance of these results, and the method of manufacture, in the production of stacked cell batteries with concomitantly higher energy densities will be highlighted.

Figure 1: Capacity retention of a 80 µAh solid state battery cycled at 1C between 3.0 and 4.0 V. Insert: Ilika micro-batteries deposited on 6” wafer

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Figure 1