טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentTiurin Ortal
SubjectCharacterization of Lif Deposited via Atomic Layer
Deposition on High-voltage LiMn1.5Nio.504
Cathode Powder
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Yair Ein-Eli


Abstract

In recent years, the growth in demand for electric vehicles (EVs) prompted extensive research in the Lithium-ion battery field to promote inexpensive, durable, stable batteries with longer range capabilities. Specifically, to increase the driving range requires the battery would have a higher energy density than the available today. To do so, several approaches are taken focusing on the capacity and voltage of the system. To increase capacity, it is common to explore anode materials such as Si, Al and Zn.

To increase the voltage, high-voltage cathode materials such as LiMn1.5Ni0.5O4 (LMNO) are researched. LMNO has a theoretical capacity of 147 mAh/gr and an operation voltage of 4.85V~, making it a desirable choice for cathode material. Nevertheless, Mn dissolution to the electrolyte during battery operation is believed to be the main cause for capacity fade over prolonged cycling. Several approaches are tried to inhibit this process, including doping of metals, use of electrolyte additives and implementation of protective coatings.           

This research focused on forming a protective coating of LiF on LMNO cathode powder using Atomic Layer Deposition (ALD).  This was carried out by using Lithium Tert-Butoxide (LiOtBut) as the Li source, alongside two different fluorine sources: Hexafluoroacetylacetone (Hfac) and TiF4. Protective characteristics of the coatings were examined via scanning electron microscopy (SEM) and inductive coupling plasma (ICP), showing improved protection against Mn dissolution.

Electrochemical performance of LiF coated LMNO powder in half-cell configuration vs Li metal revealed that both coatings had better stability during prolonged cycling. LiF coating when Hfac was the fluorine source (or LiF(Hfac)), showed higher initial capacities and higher Li content. Further examination revealed that Li diffusion occurs during deposition to form Li-Rich LMNO. The greater Li ‘reservoir’ allowed for high capacity during cycling in different c-rates up to 1.6C. In addition, it was seen using X-ray photoelectron spectroscopy (XPS) that aside from LiF formation, CFx species remain on the surface of the powder, forming a hybrid LiF-CFx layer.

As oppose to Hfac, coating with TiF4 LiF(TiF4) showed reduction in capacity vs reference, and poor c-rate performance. In addition, high over potential was observed during cycling and cyclic voltammetry (CV) measurements. Li surface deficiency was observed using time of flight secondary ion mass spectrometry (TOF-SIMS) and ICP analysis. XPS revealed that along with LiF presence, possible diffusion of fluorine into the LMNO particle also occurs. The diffusion of fluorine was suggested to stabilize Mn3 and suppression of the ion’s activity during charge/discharge, which could also contribute to the lower capacity values.

In summary, LiF coating mitigated the Mn dissolution and capacity fade of LMNO cathode powder. In addition, the effect of the ALD precursor during deposition of LiF was revealed by studying the differences between two fluorine sources: Hfac and TiF4.