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Unified ORR mechanism
In front of all the above-mentioned difficulties, it was understood that mastering the Li-O2 technology would require a deep understanding of the chemistry at play, hence calling for fundamental studies on the reaction mechanism involved. In particular, much effort has been devoted to the study of the Oxygen Reduction Reaction (ORR) since: 1) it involves oxygen reduced species responsible for most stability issues previously reported and 2) it governs the morphology of the discharge product formed during reduction that then greatly influence the charging process. Until recently, two different models had been proposed. In one hand, the model supported by Luntz describes O2 reduction into Li2O2 as a process taking place only on the surface of the electrode (2-D film, surface mechanism) 248,268. On the other hand, the model proposed by Abraham involves Li2O2 particles (3-D, solution mechanism) 206,252,269 and is based on the Hard Soft Acid Base (HSAB) theory of Pearson 270, i.e. the chemical reactivity of an acid and a base in solution. Both models were supported by experimental observations; however they are hardly compatible and have different implications regarding the rate, polarization and reversibility of Li2O2 formation/removal. In 2014, L. Johnson and C. Li proposed a unified mechanism 204 explaining both film and particle growth according to the solvents donor numbers (DN, defined as the solvation
enthalpy of the Lewis acid SbCl5 in a given solvent 271,272). Practically, a high DN solvent strongly solvates the Lewis acid Li+, which prevents its association with a base in solution such as O2¯. On the contrary, low DN solvents have low solvating power, leading to high Li+ availability. Overall, the donor number influences the equilibrium following the firsreduction step: O2 (dis) + e− ⇌ O2 − (sol).
Challenges and perspectives in non-aqueous Li-O2 batteries
For many years, rechargeable non-aqueous Li-O2 batteries were promised a great future because of their high theoretical specific energy of ~ 3500 Wh/kg (with respect to the mass of active material at the anode and at the cathode) 144. However, the initial hopes were stained by the lack of comprehensive studies requiring time and dedication. The past five years were marked by many fundamental discoveries enabling a better understanding of this unique and complex chemistry. Yet, many barriers remain before starting the development of practical Li-O2 devices.
The air electrode is soon to be a solved issue: instability problems were identified and alternative materials were proposed. Reducing their weight and cost remains challenging, but the road has been paved. The perfect electrolyte has not been found and perhaps does not exist. Trying every existing solvent has not been a very successful strategy.
Therefore, a stronger cooperation between organic chemists, battery experts and theoreticians so as to predict and synthesize this holy liquid is required. Meanwhile, the utilization of redox mediators showed promising results, and one can predict that coupling two mediators (one for the ORR, one for the OER) in the near future will enable the reversible formation of large Li2O2 without parasitic reactions and giving a large reversible capacity.
Finally, developing a suitable negative electrode is probably the most challenging issue that needs to be addressed. Both the utilization and replacement of lithium metal are problematic, mostly because of the strong oxidative operating conditions of Li-O2 cells. Owing to their high theoretical capacity, metal alloys are considered as potential candidates but currently suffer from limited cycling performances. Part of my thesis work is focused on developing an alternative anode based on lithiated silicon (LixSi). Previous studies tackling this approach were rare and elusive, hence calling for deeper investigation. Prior to consider the integration of silicon electrodes into full LixSi-O2 cells, we studied the behavior of silicon with respect to lithium in Si half cells. In order to fully understand the factors currently limiting their utilization, the next section will focus on lithium-silicon alloys as well as on Si/SiO2 composites in order to better put in context the impact of our work.
Electrochemical behavior and structural changes
First contributions on lithium-silicon alloys come from the metallurgical field 275–277. The first complete binary phase diagram 278 was described during the 80’s and mentioned the existence of 4 defined compounds: Li1,7Si (Li12Si7), Li2,3Si (Li7Si3), Li3,25Si (Li13Si4) and Li4,4Si (Li22Si4) 279–281. Meanwhile, it was reported that electrochemical lithiation of silicon at high temperature 103,104 (400-500 °C) occurred in agreement with this phase diagram 282,283, ultimately leading to the Li22Si4 phase, which corresponds to a capacity of 4200 mAh per gram of silicon. However, Li22Si4 cannot be electrochemically formed at room temperature, for which the lithiation stops at Li15Si4 284 (or Li3,75Si, 3572 mAh/gSi, 8300 mAh/cm3 Si or 3031 mAh/cm3 Li15Si4). These attractive theoretical values motivated many studies on mastering the lithium-silicon chemistry, which is particularly challenging given the massive volume changes in between the above-mentioned Li-Si phases (Figure 27). It is worth noting that owing to the linear ΔV = f(Δx) evolution, it is not possible to alleviate ΔV by shifting Δx.
Challenges and perspectives for lithium-silicon alloys
Silicon is a first-choice anode material owing to its high theoretical capacity and low potential for lithium insertion. However, it suffers from a large swelling/shrinking upon cycling, which jeopardizes its practical utilization. The development of composite electrodes containing a cohesive binder and a conductive additive highly improved the overall cycling performances. By combining a clever particle morphology and neat electrolytes additives, silicon electrodes may, in a close future, meet the standards of the industry, as it begin to be commercialized in C/Si composites.
In this work, we will use silicon as anode in Li-O2 batteries. Prior to their integration in full cells, silicon electrodes were tested in half-cells so as to optimize their cycling retention and coulombic efficiency. Through this study, we will reveal the importance, together with its understanding, of the prelithiation process in enhancing the cycle-life of Si electrodes. We will also provide a full understanding of the lithiation process of SiOx, hence opening a way to its possible utilization as additive in future Si-based composite electrodes.
Preparation of electrolytes
Numerous electrolytes were used during this work, the details of which are given below. For sake of clarity, they are all reported in Table 2 (cf p.58) annotated with their chemical formula and a few interesting properties. The electrolytes will herein be divided in two categories as function of their utilization in a) Li half-cells (which does not involve oxygen, such as Si/Li or LFP/Li cells), or b) Li-O2-type cells (which involve O2, i.e. Li/Csp/O2, LFP/Csp/O2 or LixSi/Csp/O2 cells…).
Electrolytes used in Li half-cells
Commercial LP30 (BASF, 1M LiPF6 in ethylene carbonate / dimethyl carbonate [1:1] weight ratio) was used as electrolyte in LFP/Li half cells. The manufacturer specifications are < 10 ppm of water and < 30 ppm of hydrofluoric acid (HF). If not specified otherwise, 10 wt % of FEC (Fluoroethylene Carbonate, Sigma, 99 %) was added to the LP30 when cycling or pretreating Si/Li half-cells.
Electrolytes in Li-O2-type batteries
The electrolytes used in Li-, LFP- and LixSi-O2 batteries were prepared in-house by mixing commercial salts and solvents. DMA (N,N-Dimethylacetamide, 99.8 % anhydrous, Alfa Aesar), DMSO (dimethylsulfoxide, 99.9 %, Carlo Erba), DME (1,2-dimethoxyethane, 99.9 %, Sigma Aldrich), DEGDME (diethylene glycol dimethyl ether, 99.5 % anhydrous, Sigma Aldrich) and TEGDME (tetraethylene glycol dimethyl ether, ≥ 99 %, Aldrich) were dried with activated molecular sieve (4 Å) for 3 days, so as to obtain a water content < 20 ppm as deduced by Karl Fischer titration. These last three solvents are often referred to as glyme polymers, also known as poly(ethylene oxide). In the literature, various names are used to refer to these solvents, such as monoglyme / 1G / dimethyl-PEO1 for DME, diglyme / 2G / DGME / dimethyl-PEO2 for DEGDME, and tetraglyme / 4G / TGME / dimethyl-PEO4 for TEGDME. Lithium salts LiNO3 (lithium nitrate, 99 %, Alfa Aesar) and LiTFSI (lithium Bis(trifluoromethane)sulfonimide, 99.95 %, Sigma Aldrich) were dried under vacuum at 200 and 160 °C, respectively. In contrast, LiClO4 (lithium perchlorate, battery grade, Aldrich) was used as received owing to its lower stability in temperature and sufficient purity. Appropriate solvents and salts were mixed together to prepare electrolytes of desired molarity.
Table of contents :
Chapter 1: State of the Art
I Early days of batteries
I.1 From the frog pond to the salt pond
I.2 Some lithium batteries sound better than others
II Current lithium battery technologies
II.1 Cathode materials
II.2 Anode materials
II.3 Electrolytes for Li batteries
III Post-Li-ion battery technologies
III.1 Lithium-sulfur
III.2 Non-aqueous metal-air batteries
III.3 Aqueous Li-Air batteries
IV Rechargeable Aprotic Li-O2 batteries – Last 5 years’ news
IV.1 Basic components
IV.1.a Negative electrode
IV.1.b Positive electrode
IV.1.c Electrolytes
IV.2 New trends
IV.2.a Redox mediators
IV.2.b Unified ORR mechanism
IV.3 Challenges and perspectives in non-aqueous Li-O2 batteries
V Lithium-Silicon Alloys
V.1 Electrochemical behavior and structural changes
V.2 Limiting the impacts of volume expansions
V.2.a Morphology of the particles
V.2.b Composite electrodes
V.2.c SEI and Electrolyte additives
V.2.d SiOx compounds
V.3 Prelithiation methods
V.4 Challenges and perspectives for lithium-silicon alloys
VI Conclusions
Chapter 2: Experimental procedures and new design of the Li-O2 test cell
I Material preparation
I.1 Electrodes for Li-O2 batteries
I.1.a Positive electrode
I.1.b Negative electrode
I.2 Preparation of electrolytes
I.2.a Electrolytes used in Li half-cells
I.2.b Electrolytes in Li-O2-type batteries
II Battery testing
II.1 Testing cells for Li-ion type batteries
II.1.a Two-electrode cells: Swagelok vs. Coin Cell
II.1.b Three-electrode cell
II.2 Testing cells for Li-O2 batteries
II.2.a Overview of common cells used in the literature
II.2.a.i Rudimentary cells
II.2.a.ii Metal-air dedicated cells
II.2.b Cells formerly used in our laboratory
II.2.c Cells for gas analysis
III Design of the pressurized Li-O2 test cell
III.1 Problematic
III.1.a Gas evolution monitoring
III.1.b Reproducibility
III.1.c User-friendliness
III.2 Cell description
III.3 Pressure measurement
III.3.a Sensor connection
III.3.b Sensor calibration
III.4 Related equipment
III.4.a Filling station
III.4.b Temperature controlled chamber
IV Figures of merit of the pressurized cell
IV.1 Stability
IV.2 Sensitivity
V Conclusions
Chapter 3: Study of lithium-oxygen batteries using a pressurized electrochemical test cell
I Prologue
I.1 Data interpretation
I.2 Added value of Pressurized Cells for studying Li-O2 batteries
II Lithium Nitrate in N,N-Dimethylacetamide (DMA)
III Tetraethyleneglycol dimethylether (TEGDME)
III.1 Electrochemical behavior
III.2 Identification of the parasitic reactions
III.2.a Comparing DMA- and TEGDME-based systems by impedance spectroscopy .
III.2.b Parasitic reactions at the positive and negative electrodes
III.2.c Mitigating parasitic reactions using redox mediators
IV Influence of the glyme chain length
IV.1 DME vs. longer glymes
IV.2 Redox mediator in DME-based cells
V Dimethyl Sulfoxide (DMSO)
V.1 Quantification of parasitic reactions
VI Conclusions
Chapter 4: Development of Si composite electrodes as anode in LixSi-O2 batteries
I Si composite electrode with good cycling performance
I.1 Composite electrode preparation
I.2 Influence of the particle size on the cycling performances
I.3 Improving the cycling retention of M-Si-based electrodes
I.4 Improving the capacity of Si NP-based electrodes
II Influence of the pre-lithiation onto Si NP and their SiO2 shell
II.1 Electrochemical pre-lithiation techniques
II.2 Improved performances of prelithiated Si NP electrodes
II.2.a Effect of a short-circuit and a plating sequence
II.2.b Investigating the SiO2 reduction process using a potentiostatic discharge
II.3 Reduction of pure silica
III Lithium-Air batteries using lithiated silicon as anode
III.1 The LixSi electrodes within the context of Li-Air batteries
III.1.a Electrode loading
III.1.b Balancing Li losses
III.1.c Prelithiation sequence
III.2 Recovering the LixSi electrode
III.2.a Resting time after lithiation
III.2.b Glass Fiber vs. Celgard-type separator
III.2.c Washing process and cycling in a Li-O2 electrolyte
III.3 Study of full LixSi-O2 batteries
III.3.a Experimental setup
III.3.b LixSi-O2 full cells using high capacity Si electrodes
III.3.c LixSi-O2 full cells using p-LixSi electrodes
III.3.d LixSi-O2 full cells with limited depth of discharge
III.3.e Improving the cycle-life of the LixSi anode with a physical protection
IV Conclusions
General Conclusions
Bibliography
