3D Nanocomposite Thin Film Cathodes for Micro‐Batteries with Enhanced High‐Rate Electrochemical Performance over Planar Films

Abstract

High energy and high power density rechargeable micro‐batteries are a necessity for powering the next generation of flexible electronics, internet of things, and medical technology devices. In theory, significant improvements in the capacity, current and power densities of micro‐batteries would result if 3D architectures with enhanced interdigitated component interface areas and shortened ion diffusion path‐lengths were used. Further gains are achievable if the materials utilized have high crystalline quality and are preferentially oriented for fast lithium intercalation. In this work, this is achieved by creating epitaxial thin film cathodes comprised of nanopillars of LiMn2O4 (LMO) embedded in a supporting matrix of electronically conducting SrRuO3 (SRO). The first electrochemical study of such a 3D vertically aligned nanocomposite (VAN) that displays clear cathode redox signatures is provided, and demonstrates remarkable capacity retention under high‐rate regimes. The electrochemical performance is shown to be dependent on the nanopillar topography, namely the crystallographic orientation, nanopillar dimensions, and electrode/electrolyte interfacial surface area. This work offers a pathway to realizing 3D architectured micro‐batteries with high‐capacity retention under high‐rate conditions enabling fast charge capabilities.

Stabilizing Crystal Framework of an Overlithiated Li1+xMn2O4 Cathode by Heterointerfacial Epitaxial Strain for High-Performance Microbatteries

“…The film area is the size of the substrate (5 × 5 mm). Theoretical density of cubic LiMn 2 O 4 is 4.28 g cm –3 . − …”

“…The 100/110-films at “discharged 2.5 V” and “charged 3.6 V” were prepared by stopping the reduction scan at 2.5 V and the oxidation scan at 3.6 V, respectively, after five cycles. The calculations of the gravimetric capacities are based on eqs and : − .25em Film mass .25em ( g ) = Film thickness .25em ( cm ) × Film area .25em ( cm 2 ) × Theoretical density .25em ( g cm − 3 ) Gravimetric capacities .25em ( mAh g − 1 ) = Measured capacity .25em ( mAh ) Film mass .25em ( g ) …”

“…The 100/110-films at “discharged 2.5 V” and “charged 3.6 V” were prepared by stopping the reduction scan at 2.5 V and the oxidation scan at 3.6 V, respectively, after five cycles. The calculations of the gravimetric capacities are based on eqs and : − …”

“…Epitaxial thin film growth on a single crystalline substrate allows the precise control of the crystal orientation of a material based on the substrate orientation and its interface structure adjacent to the underlying substrate. So far, several epitaxial LMO thin film model systems have been fabricated to elucidate valuable insight into the potential degradation mechanism in LMO. − Moreover, (111) surface-dominated epitaxial LMO thin films with different crystal orientations have been investigated at the 4 V region to demonstrate enhanced high-rate electrochemical behavior and extended cycle life without excessive capacity fading. , Recently, the low-temperature synthesis of epitaxial LMO films was enabled by the usage of a NiCo 2 O 4 current collector . However, a stable, reversible overlithiation of an epitaxial LMO thin film cathode involving both 4 and 3 V regions is still lacking, which limits the implementation of thin film cathodes in high-performance microbatteries.…”

Lithium Loss in Vacuum Deposited Thin Films

“…5 If the growth temperature is too high (>∼650 °C), lithium can be completely driven off resulting in the spinel LMO phase not forming. 41 Conversely, if the growth temperature is significantly lowered, this can result in the formation of lithium-rich LMO, including Li 2 Mn 2 O 4, compensating for volatility issues. 42 For the solid-state electrolytes Li 3x La 2/3−x TiO 3 (LLTO) and Li 7 La 3 Zr 2 O 12 (LLZO), lowered Li + ionic conductivity, typically 2−3 orders of magnitude lower in thin films vs bulk, is often reported and ascribed to lithium loss (Figure 2e,f).…”