Periodic Reporting for period 1 - HS-GLASSion (Highly stable glasses applied for lithium ion battery electrolytes)
Reporting period: 2015-08-01 to 2017-07-31
One of the main challenges in rechargeable energy sources is providing high safety, high energy and power density with reliable cycling performance and operational stability. Rechargeable Lithium ion batteries (rLiBs) are one of the most frequent types of cells employed in portable electronics due to their superior specific power and energy. Typical rLiBs make use of liquid electrolytes. However, these devices have several disadvantages, like the need of separators that limits their downscaling and the high risk of fire and explosion due to leakage. Thus, suitable solid electrolytes are needed to overcome these drawbacks. Inorganic glass-based electrolytes are very promising materials due their ease of preparation and can be easily downscalable which can provide an advantage due to short Li+ diffusion lengths. The demand for flexible and lighter batteries with large energy densities and power output is continuously increasing. The growth in thin film technology and miniaturization applications (i.e. IoT) gave a significant boost to this field. The use of all solid-state batteries (ASSB) could surmount the limitations of commonly used Li-ion batteries employing liquid electrolytes. Also, ASSB can explore other routes for battery integration that typical liquid electrolyte batteries cannot, such as form factor, scalability and material property control. The introduction of these type of new devices will be helpful to fulfill the needs of miniaturized power sources for modern society. The main purpose of HS-Glass+ion is to find ways of obtaining highly stable glass-based (HSG) electrolytes that can enhance thermal stability, ion conduction and improve interfacial effects, integrate them in thin film ASSB and systematically study its effects on device performance. In this project, thin film solid state electrolytes are grown to study the deposition temperature dependence on ionic conductivity, battery performance and stability. The structural changes in these materials due to process conditions will definitely modify their performance and affect the overall performance on a complete thin film battery device in a way that has not yet been explored so far in the field of thin film ASSB. To test these possible effects, several fully working thin film batteries were fabricated in HS-Glass+ion. The battery devices were based on Li4Ti5O12 (LTO) cathode material electrode, Lithium Phosporous Oxinitride (LiPON) solid electrolyte layers with different properties and Lithium metal as anode electrode. These batteries give an open circuit potential of 1.5 V. These thin film ASSB were characterized in order to determine their performance and dependence on LiPON properties. The reported outcome is interpreted as the result of a change in the local structure of the glassy network of LiPON which makes Li diffusion through vacancies and interstitials tougher to occur. This is related to the creation of a HSG LiPON layer with a closed-packed structure which is attained at deposition temperatures close to the glass transition temperature (Tg) of the material.
Solid electrolyte LiPON layers were deposited by RF-sputtering. Different substrate temperatures (Tdep) were employed ranging from RT to 350 oC. The dependence upon Tdep to elemental composition, structure, ionic conductivity and thermal stability were established. The composition of LiPON layers was determined by elastic recoil detection (ERD). X-ray diffraction (XRD) was performed to determine the structure of the films. Ionic conductivity values were extracted by Solid-state Impedance Spectroscopy (SIS). To obtain information on the thermal stability, in-situ XRD and spectroscopic ellipsometry were used to identify the crystallization (Tc) and glass transition (Tg) temperature, respectively. LiPON layers retain their amorphous character even when deposited at a temperature of 285 oC. The composition does not change upon Tdep. Ionic conductivity decreases with increasing Tdep by almost 2 orders of magnitude. Tg is determined to be 219 oC. As Tdep increases the onset of the Tg shifts to higher temperatures implying an enhancement in thermal stability. This hypothesis is corroborated since Tc shifts to higher values as Tdep is increased. Therefore, LiPON layers with a higher resistance to crystallization can be obtained. All of the results obtained within HS-Glass+ion give insight and useful information to correlate the structural characteristics of solid electrolytes to ionic motion within its structure. Changes seen in material properties and previous reported literature indicate that there are structural modifications of LiPON layers as the Tdep increases. These changes go in hand with the concept of HGS. The proof of this concept to this type of materials was one of the main objectives of HS-Glass+ion. These overall effects were tested in fully functional thin film ASSB for the first time. The reduced ionic conductivity of high temperature LiPON layers is shown to clearly affect the capacity and c-rate performance of the full thin film batteries. The maximum attainable capacities are decreased when using high temperature LiPON. LTO thin film electrodes were deposited by RF-sputtering and used as cathode. Subsequently the LiPON solid electrolyte is deposited at different temperatures. Finally, Lithium metal anode is thermally evaporated completing the full battery stack. Cyclic voltammetry (CV) and galvanostatic charge-discharge experiments were carried out. The c-rate performance was determined and was found to be in good relation with the lowering of ionic conductivity. These results show that battery performance is largely affected by the LiPON solid electrolyte properties and by the possible interfacial effects. The performance of thin film batteries can be drastically altered by having structural variations in the solid electrolyte film. Although ionic conductivity decreases worsening battery behavior, its thermal stability drastically enhances which might open new possibilities for high temperature applications. General efforts to go beyond HS-Glass+ion and the understanding of thin film solid electrolyte stability capability resulted in 1 accepted and 3 submitted manuscripts including oral/poster presentations in 3 conferences. Another manuscript studying in detail the stability of these films is under preparation. Related to this work, 2 master thesis topics were developed and were graded the highest scores from their respective institutions.
The breakthrough of highly stable glasses has clearly impacted the fundamental study of amorphous materials and current technologies that use glasses, such as flexible and organic electronics. Within HS-Glass+ion it has been demonstrated that LiPON solid electrolyte can achieve a highly stable state throughout deposition at higher temperatures. The results show that higher thermal stability can be achieved in LiPON where the crystallization (Tc) and glass transition (Tg) temperature are shifted to higher values. This is the first time where the concept of highly stable glasses is introduce in battery research. Although ionic conductivity values are reduced this concept may open new application scenarios where high temperature stability is needed. HS-GLASS+ion has contribute to advance state-of-the-art with the use of HSG’s in solid state LIB’s towards enhancing device performance. Devices that will be for sure used in consumer electronics in years to come.