Brecht Put

High Cycling Stability and Extreme Rate Performance in Nanoscaled LiMn2O4 Thin Films

Ultrathin LiMn2O4 electrode layers with average crystal size of ∼15 nm were fabricated by means of radio frequency sputtering. Cycling behavior and rate performance was evaluated by galvanostatic charge and discharge measurements. The thinnest films show the highest volumetric capacity and best cycling stability, retaining the initial capacity over 70 (dis)charging cycles when manganese dissolution is prevented. The increased stability for film thicknesses below 50 nm allows cycling in both the 4 and 3 V potential regions, resulting in a high volumetric capacity of 1.2 Ah/cm3. It is shown that the thinnest films can be charged to 75% of their full capacity within 18 s (200 C), the best rate performance reported for LiMn2O4. This is explained by the short diffusion lengths inherent to thin films and the absence of phase transformation.

Electrical Characterization of Ultrathin RF-Sputtered LiPON Layers for Nanoscale Batteries

Ultrathin lithium phosphorus oxynitride glass (LiPON) films with thicknesses down to 15 nm, deposited by reactive sputtering in nitrogen plasma, were found to be electronically insulating. Such ultrathin electrolyte layers could lead to high power outputs and increased battery energy densities. The effects of stoichiometry, film thickness, and substrate material on the ionic conductivity were investigated. As the amount of nitrogen in the layers increased, the activation energy of the ionic conductivity decreased from 0.63 to 0.53 eV, leading to a maximum conductivity of 1 × 10(-6) S/cm. No dependence of the ionic conductivity on the film thickness or substrate material could be established. A detailed analysis of the equivalent circuit model used to fit the impedance data is provided. Polarization measurements were performed to determine the electronic leakage in these ultrathin films. A 15-nm LiPON layer on a TiN substrate showed electronically insulating properties with electronic resistivity values around 10(15) Ω·cm. To our knowledge, this is the thinnest RF-sputtered LiPON layer shown to be electronically insulating while retaining good ionic conductivity.

On the chemistry and electrochemistry of LiPON breakdown

Battery safety and cycle life remain of concern to current Li-ion batteries that use a liquid organic electrolyte. Both issues can be resolved by employing a solid electrolyte. Numerous Li-ion conducting solids are known today, however the stability of most of these is too low to engender widespread usage. Here, we report on the decomposition chemistry and electrochemistry of LiPON, at present the most commonly used solid electrolyte. The decomposition potential was calculated from basic thermodynamic quantities and was linked to current–voltage (I–V) measurements with great consistency. The decomposition of LiPON was shown to occur at a potential of 4.3 V and proceeds in a diffusion limited way. The LiPON layer decomposes into Li-ions, O2 or N2 gas and a phosphate rich compound. Ultimately hard breakdown could occur through the formation of metallic lithium filaments that short circuit the LiPON layer as shown by TOF-SIMS imaging experiments. As such the work provided here also provides insight in the breakdown mechanism of RRAM devices.

Plasma-assisted and thermal atomic layer deposition of electrochemically active Li2CO3

Thin-film lithium carbonate (Li2CO3) has applications in various electrochemical devices, like Li-ion batteries, gas sensors and fuel cells. ALD of Li2CO3 is of interest for these applications as it allows for uniform and conformal coating of high-aspect ratio structures and particles with very precise thickness control. However, there are few studies that focus on its fabrication and characterization. In this work, plasma-assisted and thermal ALD were adopted to grow ultra-thin, conformal Li2CO3 films between 50 and 300 °C using lithium tert-butoxide as a precursor and O2 plasma or H2O/CO2 as co-reactants. More specifically, we focus on the plasma-assisted process by film growth, stability and conductivity studies and emphasize the differences from its more extensively adopted thermal counterpart. Plasma-assisted ALD allows for higher growth per cycle values (0.82 vs. 0.60 A), lower substrate temperatures and shorter cycle times. The stoichiometry of the films, ranging from Li2CO3 to Li2O, can be controlled by substrate temperature and O2 plasma exposure time. The ionic conductivity for both plasma-assisted and thermal ALD is measured for the first time and is in the order of 10−10 S cm−1 after normalizing to the different effective surface areas. The Li-ion conductivities found here are in line with literature values predicted by simulation studies.