  Thin film solid state lithium-based batteries (TSSBs) are increasingly
attractive for their intrinsic safety due to the use of a nonflammable solid
electrolyte, cycling stability, and ability to be easily patterned in small
form factors. However, existing methods for fabricating TSSBs are limited to
planar geometries, which severely limits areal energy density when the
electrodes are kept sufficiently thin to achieve high areal power. In order to
circumvent this limitation, we report the first successful fabrication of fully
conformal, 3D full cell TSSBs formed in micromachined silicon substrates with
aspect ratios up to ~10 using atomic layer deposition (ALD) at low processing
temperatures (at or below 250C) to deposit all active battery components. The
cells utilize a prelithiated LiV$_2$O$_5$ cathode, a very thin (40 - 100 nm)
LiPON-like lithium polyphosphazene (Li$_2$PO$_2$N) solid electrolyte, and a
SnN$_x$ conversion anode, along with Ru and TiN current collectors. Planar
all-ALD solid state cells deliver 37 {\mu}Ah/cm$^2${\mu}m normalized to the
cathode thickness with only 0.02% per-cycle capacity loss for hundreds of
cycles. Fabrication of full cells in 3D substrates increases the areal
discharge capacity by up to a factor of 9.3x while simultaneously improving the
rate performance, which corresponds well to trends identified by finite element
simulations of the cathode film. This work shows that the exceptional
conformality of ALD, combined with conventional semiconductor fabrication
methods, provides an avenue for the successful realization of long-sought 3D
TSSBs which provide power performance scaling in regimes inaccessible to planar
form factor devices.
