(Invited) Conformal Energy Storage: Atomic Layer Deposition of 3D Solid State Batteries
Authors: Pearse Alexander J , Gregorczyk Keith , Stewart David , Rubloff Gary 
Published: February 2020

The number of electrochemically active films available for growth via atomic layer deposition (ALD) has exploded in recent years, and now includes solid state ionic conductors, anodes, cathodes, current collectors, protection layers and more. The next frontier is to combine these materials into multifunctional, active devices on complex or three-dimensional (3D) topography, taking advantage of ALD’s conformality.

In certain cases, 3D fabrication enables new regimes of device performance. Thin film solid state batteries (SSBs) are an attractive energy storage technology due to their intrinsic safety, stability, and tailorable form factor. However, as thin film SSBs are typically fabricated only on planar substrates by line-of-sight deposition techniques, their areal energy storage capacity (< 1 mWh/cm2) and application space is limited. Moving to three dimensional architectures provides fundamentally new opportunities in power/energy areal density scaling, but requires a new fabrication process. In this talk, we will describe the development of a 3D solid state battery which is grown entirely by ALD.

We describe the development of several novel ALD processes for enabling battery components, including a LiPON-family lithium solid electrolyte grown using the precursors lithium tert-butoxide and diethyl phosphoramidate. The ALD reaction forms the amorphous product Li2PO2N, which is determined to be a promising solid electrolyte with an ionic conductivity of 6.5 × 10-7 S/cm at 35C and wide electrochemical stability window of 0-5.3 V vs. Li/Li+. The ALD LPZ is integrated into a variety of solid state batteries to test its compatibility with common electrode materials, including LiCoO2 and LiV2O5, as well as flexible substrates. Planar solid state batteries operate with extraordinarily thin (< 40 nm) solid state electrolytes, lowering the net resistance to values competitive with the best sputtered LiPON. We additionally develop and characterize high-capacity conversion anode materials in the tin oxynitride family, finding that nitrogen incorporation improves the performance of the typically short-lived conversion material.

These materials are combined with a LiV2O5 (LVO) cathode and ALD-grown current collectors to form a stable all-ALD solid state battery film stack grown entirely at or below 250C. 3D cells are formed by growing the film stack in micromachined Si hole arrays combined with ion-milling isolation of individual cells. The full cell exhibits a reversible capacity of ~35 μAh cm-2 μmLVO
-1 with an average discharge voltage of ~2V. By growing the batteries into 3D arrays of varying aspect ratios, we demonstrate upscaling the areal capacity of the battery by approximately one order of magnitude while simultaneously improving the rate performance and round-trip efficiency, in a regime for which this improvement is impossible for a planar configuration as indicated by finite element simulations of lithium transport. Finally, we discuss projections for how this proof-of-concept device could be extended to performance numbers approaching or exceeding the areal power densities of conventional Li-ion technology.