3 research outputs found
Nanoscale Solid State Batteries Enabled by Thermal Atomic Layer Deposition of a Lithium Polyphosphazene Solid State Electrolyte
Several
active areas of research in novel energy storage technologies,
including three-dimensional solid state batteries and passivation
coatings for reactive battery electrode components, require conformal
solid state electrolytes. We describe an atypical atomic layer deposition
(ALD) process for a member of the lithium phosphorus oxynitride (LiPON)
family, which is employed as a thin film lithium-conducting solid
electrolyte. The reaction between lithium <i>tert</i>-butoxide
(LiO<sup>t</sup>Bu) and diethyl phosphoramidate (DEPA) produces conformal,
ionically conductive thin films with a stoichiometry close to Li<sub>2</sub>PO<sub>2</sub>N between 250 and 300 °C. Unusually, the
P/N ratio of the films is always 1, indicative of a particular polymorph
of LiPON that closely resembles a polyphosphazene. Films grown at
300 °C have an ionic conductivity of (6.51 ± 0.36) ×
10<sup>–7</sup> S/cm at 35 °C and are functionally electrochemically
stable in the window from 0 to 5.3 V versus Li/Li<sup>+</sup>. We
demonstrate the viability of the ALD-grown electrolyte by integrating
it into full solid state batteries, including thin film devices using
LiCoO<sub>2</sub> as the cathode and Si as the anode operating at
up to 1 mA/cm<sup>2</sup>. The high quality of the ALD growth process
allows pinhole-free deposition even on rough crystalline surfaces,
and we demonstrate the successful fabrication and operation of thin
film batteries with ultrathin (<100 nm) solid state electrolytes.
Finally, we show an additional application of the moderate-temperature
ALD process by demonstrating a flexible solid state battery fabricated
on a polymer substrate
Physical Delithiation of Epitaxial LiCoO<sub>2</sub> Battery Cathodes as a Platform for Surface Electronic Structure Investigation
We report a novel delithiation process for epitaxial
thin films
of LiCoO2(001) cathodes using only physical methods, based
on ion sputtering and annealing cycles. Preferential Li sputtering
followed by annealing produces a surface layer with a Li molar fraction
in the range 0.5 x < 1, characterized by
good crystalline quality. This delithiation procedure allows the unambiguous
identification of the effects of Li extraction without chemical byproducts
and experimental complications caused by electrolyte interaction with
the LiCoO2 surface. An analysis by X-ray photoelectron
spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides
a detailed description of the delithiation process and the role of
O and Co atoms in charge compensation. We observe the simultaneous
formation of Co4+ ions and of holes localized near O atoms
upon Li removal, while the surface shows a (2 × 1) reconstruction.
The delithiation method described here can be applied to other crystalline
battery elements and provide information on their properties that
is otherwise difficult to obtain
Imaging Phase Segregation in Nanoscale Li<sub><i>x</i></sub>CoO<sub>2</sub> Single Particles
LixCoO2 (LCO)
is a common
battery cathode material that has recently emerged as a promising
material for other applications including electrocatalysis and as
electrochemical random access memory (ECRAM). During charge–discharge
cycling LCO exhibits phase transformations that are significantly
complicated by electron correlation. While the bulk phase diagram
for an ensemble of battery particles has been studied extensively,
it remains unclear how these phases scale to nanometer dimensions
and the effects of strain and diffusional anisotropy at the single-particle
scale. Understanding these effects is critical to modeling battery
performance and for predicting the scalability and performance of
electrocatalysts and ECRAM. Here we investigate isolated, epitaxial
LiCoO2 islands grown by pulsed laser deposition. After
electrochemical cycling of the islands, conductive atomic force microscopy
(c-AFM) is used to image the spatial distribution of conductive and
insulating phases. Above 20 nm island thicknesses, we observe a kinetically
arrested state in which the phase boundary is perpendicular to the
Li-planes; we propose a model and present image analysis results that
show smaller LCO islands have a higher conductive fraction than larger
area islands, and the overall conductive fraction is consistent with
the lithiation state. Thinner islands (14 nm), with a larger surface
to volume ratio, are found to exhibit a striping pattern, which suggests
surface energy can dominate below a critical dimension. When increasing
force is applied through the AFM tip to strain the LCO islands, significant
shifts in current flow are observed, and underlying mechanisms for
this behavior are discussed. The c-AFM images are compared with photoemission
electron microscopy images, which are used to acquire statistics across
hundreds of particles. The results indicate that strain and morphology
become more critical to electrochemical performance as particles approach
nanometer dimensions