Electrochemistry

Matthew Lacey (Uppsala University, Sweden) gave a talk about water-based electrodes and more particularly about the effects of polyethylene oxide (PEO) as an electrolyte additive or as a binder. His group prepared a carbon-sulfur material by melting carbon (Ketjenblack^® EC-600JD) and sulfur under air at 150°C. For the water based electrode they added C65 and 4:1 PEO:polyvinylpyrrolidone (PVP). PEO swells and locally changes the solvent system to the benefit of polysulfide (PS) solubility and kinetics. PVP is soluble but forms an insoluble complex with Li₂S_x . Ketjenblack^® is the most challenging carbon adhesion-wise but shows the best electrochemical performance. 1000 mAh g^–1 was obtained for 50 cycles using electrodes with a S loading of 2 mg cm^–2. Their conclusion is that PEO:PVP is a suitable water-based binder for electrodes based on highly porous C black.

Diana Golodnitsky (Tel Aviv University, Israel) gave a talk about the strategies to impede PS shuttle by using barrier membranes between cathode and separator or by depositing a thin barrier over them. Toray and SGL carbon papers are the barrier membranes used which are also electrochemically active. Cells containing these barrier membranes delivered high S utilisation and excellent columbic efficiency, which suggests that the PS shuttle has been reduced. Electrodes or cathodes deposited with thin PEO-XC72 or lithium tin phosphorous sulfide (LSPS)-PEO based layers also showed this positive effect. However, the cells with barrier layers had capacity fading problems.

Markus Hagen (Fraunhofer Institute for Chemical Technology (ICT), Germany) studied electrolyte decomposition. This decomposition is monitored by fitting a pressure sensor and applying mass spectroscopy (MS) to a LiS cell. The pressure increases during discharge due to the release of gases such as carbon disulfide (CS₂) and nitrogen, produced by the decomposition of the electrolyte. N₂ gas is produced due to the decomposition of lithium nitrate (LiNO₃) in the electrolyte, an additive which is widely used to suppress the PS redox shuttle. In contrast, electrolytes without it did not show any N₂ gas evolution; therefore, the speaker concluded that LiNO₃ is not beneficial since it promotes decomposition of the electrolyte.

Characterisation Techniques

Alice Robba (French Alternative Energies and Atomic Energy Commission (CEA), Innovation Laboratory for the Technologies of New Energies and Nanomaterials (LITEN), France) gave an interesting talk about Li₂S particle size influence on the first charge working mechanism of lithium sulfide (Li₂S) based Li-ion batteries. In order to overcome the safety issue with the use of Li metal in a LiS system, her group used Li₂S as a positive electrode and silicon as a negative electrode. Replacing S₈ by Li₂S as the active material allows the use of a safer negative electrode. However, S₈ and Li₂S have different conductivity and solubility properties. The focus of the work presented was the effect of Li₂S particle size on the charge mechanism of Li₂S based Li-ion batteries. When Li₂S was prepared by electrodeposition (nano-Li₂S), two plateaus were visible for S₈ evolution (≈2.3 V and 2.45 V). However, when commercial Li₂S (micro-Li₂S) was used, a huge difference was seen in the electrochemical signature. In order to understand this difference between nano-Li₂S and micro-Li₂S, the respective batteries were analysed by operando X-ray absorption and emission spectroscopies coupled with operando X-ray diffraction. When nano-Li₂S was used, a smaller polarisation and a smaller activation barrier for Li₂S oxidation were observed. Operando studies showed a very nice PS evolution and a complete disappearance of Li₂S. However, when micro-Li₂S was used, a higher polarisation was seen upon charging and there was no PS detection while E>3.3 vs. Li/Li⁺. There was consumption of Li₂S but never a total disappearance and there was an early appearance of S. Therefore, Robba’s conclusion was that use of nano-Li₂S is better because the polarisation can be lowered and there is a smaller ‘activation’ barrier for Li₂S oxidation.

Céline Barchasz (CEA LITEN) gave an overview of different types of LiS systems. In a Li-ion/S system the cathode initially contains Li₂S and the anode is graphite, which has better cyclability since the parasitic reactions involving Li metal anode are prevented. In a catholyte cell, comprising a high concentration (7 M) of PS in electrolyte, a special current collector with vertical C fibres is used. However the specific energy is limited to only –1 due to the limited solubility of PS. For encapsulated S in a C micropore system a pseudo-solid-state reaction takes place, where the signature of the galvanostatic discharge curve is different from the conventional Li/S₈ cell (1). Barchasz also talked about the tomography studies done in her group. An ex situ X-ray phase contrast tomography technique (2) was used to characterise the evolution of the cathode morphology upon cycling and to track the loss of S from the conductive C network. A special cell was used for the operando X-ray absorption tomography which consists of an aluminium can that houses the Li-metal anode, a separator containing electrolyte and a C paper as the cathode. The solid S₈ was initially placed over the C paper, which disappears during discharge and redistributes throughout the cathode during the subsequent charging sequence. The images were two-dimensional (2D) cross sections of the cell and each constituent material was shown in a different grey-scale.

Due to their high stability with no risks of vaporisation and leakage, solid-state batteries are a promising alternative to liquid electrolytes. Jessica Lefevr (Technical University of Denmark) gave a talk about solid electrolytes for LiS systems based on lithium borohydride (LiBH₄), which is an interesting salt for LiS due to its light weight and its electrochemical stability. While the orthorhombic phase (Pnma ) is stable at room temperature and has a low ionic conductivity (10^–5 mS cm^–1 at 30°C), the hexagonal phase (P6₃/mmc ), which is stable above 110°C, has a much higher ionic conductivity (1 mS cm^–1 at 120°C). Confinement of BH₄ in mesoporous silicon dioxide (SiO₂) allows fast ionic conductivity even at room temperature. Operando X-ray diffraction, tomography and Raman spectroscopy measurements were used to understand the electrochemistry and improve the batteries’ performance.

Deyang Qu (University of Wisconsin Milwaukee, USA) reported an in situ characterisation technique in which electrolyte retrieved from the cell at different states of charge (SOC) were analysed using high performance liquid chromatography (HPLC) and MS. 4-(dimethylamino)benzoyl chloride (3) is added to the electrolyte prior to HPLC and MS measurements, in order to prevent the dissolved PS in them from undergoing disproportionation reactions. HPLC is used to separate different PS and MS is used to identify their speciation. Based on the in situ HPLC and MS results Qu has proposed reaction mechanisms for both discharge and charge processes. According to him, the two plateaus characteristic of the charge and discharge curves arise due to the equilibrium between different PS ions.

Applications

For aerospace applications, gravimetric energy density is more important than the volumetric energy density. NASA and Airbus are currently using LiS technology and both gave a talk about the importance of the research in this field. The challenge is still to have higher S loading (m_(S) = 13 mg cm^–2 ideally).

The Zephyr aircraft is a high-altitude long-endurance unmanned aerial vehicle that holds the world record for the longest flight duration of any aircraft: 14 days non-stop. It has been in development since 2004 and uses LiS batteries to store the energy captured by its solar panels and power the aircraft at night. Sarah Bassett (Airbus Defence and Space) gave a talk about the model ‘Zephyr T’ that is under development. This model will provide communications and internet connectivity to developing world countries without the need for installing expensive infrastructure on the ground. She insisted on the need for ultra-lightweight batteries and explained why LiS is perfect for this application.

Conclusion

LiS batteries are a most promising candidate for the next generation of electrical energy storage. They are made of cheap and abundant elements and they have a theoretical specific energy of ca. 2500 Wh kg^–1 or 2800 Wh l^–1, which is much higher than that of Li-ion batteries (Figure 1). Yet their cycling stability shows limitations and scaled production technologies need to be established to meet cost targets.

The LiSM³ was a very interesting conference where many issues with LiS were discussed. LiS is of interest where light weight is important such as in aircraft. The main current research activity is to control the PS via the development of new separators and new electrolyte formulations. Anode protection was often mentioned as a key challenge (often in the conclusions slide) but no one gave a talk about it. Very little was about the cathode preparation. Engineering is also crucial for the development of this technology.