Effect of Cr-oxide partial coating on the electrochemical behavior of thin film high-voltage
spinel
E. Garcia-Tamayo, J. Ros, A. Kaas, R. Fredon, E.M. Kelder
TU Delft, ChemE, Nanostructured Materials Julianalaan 136 2628 BL, Delft, The Netherlands The stored energy content of a (Li ion) battery can be optimized by increasing the specific capacity (mAhg-1) of the cell, maximizing the potential difference between positive (cathode) and negative (anode) electrodes and by reducing the amount of inactive material.
Lithium nickel manganese spinel (LiNi0.5Mn1.5O4)
presents a high potential of 4.9 V versus lithium, compared to the 3.7 V for the LiCoO2 which is one of the
standard cathode materials in today’s Li ion batteries. Electrochemical testing under high rates, for both ordered and disordered structure of this material, has been studied for nano- and micron-sized particles [1, 2]. Results suggest that ion or electron transport in the particle is not the rate-limiting process, instead the performance of the material is mainly obstructed by the lithium transfer properties from the electrolyte into the bulk. Hence, the surface plays a crucial role in the kinetics.
Chromium oxide has been widely studied due to its importance in industrial applications, which includes coating of materials for high temperature uses, wear resistance for tribological applications and, of particular interest for this study, its catalytic applications. Moreover, it has been used in battery applications where Cr-oxide coating of LiMn2O4 increased Mn4+ concentration at the
surface and prevented the direct contact between the active particles and electrolyte, reducing the dissolution of manganese and the oxidation of the electrolyte [3].
Electrostatic Spray Pyrolysis is a process in which thin layers are synthesized and deposited in the same step. A high electric field is applied to the surface of a liquid precursor flowing through a narrow capillary, forming a fine aerosol with droplets with sizes smaller than 1μm diameter. These droplets are guided onto a heated substrate where pyrolysis occur producing films with different properties and of a wide range of materials [4]. Density, morphology and thickness can be controlled by tuning the parameters of both precursor solution and ESP setup.
EXPERIMENTAL
In this work LiNi0.5Mn1.5O4 thin films were prepared
using the Electrostatic Spray Pyrolysis (ESP) method, as described in previous works [5, 6]. Nickel nitrate hexahydrate, lithium nitrate anhydrous, and manganese nitrate tetrahydrate, where dissolved in 2-propanol with a concentration of 0.1 M, in stochiometric amounts.
Cr-oxide partial coating was carried out in two different ways: by a dip coating process at different immersion times on chromium (III) nitrate aqueous solution with different concentrations and via electrospray pyrolysis of a 0.1 M chromium nitrate nonahydrate dissolved in 2-propanol at 400°C, using different deposition times. Further annealing at 500°C was done for both processes. XRD was performed in order to study the structure and composition. The electrochemical performance of the film was studied by galvanostatic measurements using metallic Li as reference and counter electrode in a 1M LiPF6 EC:DMC (1:1 by wt.) electrolyte.
SEM and EDX were used to determine the morphology
and composition of the films.
RESULTS AND DISCUSSION
Preliminary SEM micrographs of sprayed Cr-oxide with different times of deposition show an increase of coverage of Cr2O3 structures over a flat and dense
LiNi0.5Mn1.5O4 layer.
Figure 1. SEM images of ESP as-deposited Cr2O3 spots
for 1 min (upper left), 5 min (upper right) and 10 min (lower middle).
HIGHLIGHTS
An in depth discussion on the relations between the morphology, deposition time and electrochemical performance will be presented during the conference. Special attention will be paid on a novel method to measure the kinetics via a silicon AFM cantilever that is prelithiated in an electrochemical AFM cell. The cell is constructed such that the LiNi0.5Mn1.5O4 surface can be
monitored with the AFM tip near the deposited Cr-oxide parts, so as to obtain local enhanced conductivity, e.g. power performance. The cell has an additional lithium electrode as reference and for further lithiation of the silicon tip.
REFERENCES
[1] G. Ceder et al., J. Electrochem. Soc., 2010, 157, 8, A925-A931.
[2] P. Bruce et al., Dalton Trans., 2008, 5471.
[3] H. Sahan et al., Solid State Ionics, 2010, 181, 1437. [4] Jaworek, A. et al., J. Mater. Sci., 2007, 42, 266-297. [5] U. Lafont et al., Thin Solid Films, 2012, 520 3464–
3471.
[6] E. Garcia-Tamayo et al., J. Power Sources, 2011, 196, 6426.
