Li 2 S is a high-capacity cathode material for lithium metal-free rechargeable batteries. It has a theoretical capacity of 1166 mAh/g, which is nearly 1 order of magnitude higher than traditional metal oxides/phosphates cathodes. However, Li 2 S is usually considered to be electrochemically inactive due to its high electronic resistivity and low lithiumion diffusivity. In this paper, we discover that a large potential barrier (∼1 V) exists at the beginning of charging for Li 2 S. By applying a higher voltage cutoff, this barrier can be overcome and Li 2 S becomes active. Moreover, this barrier does not appear again in the following cycling. Subsequent cycling shows that the material behaves similar to common sulfur cathodes with high energy efficiency. The initial discharge capacity is greater than 800 mAh/g for even 10 μm Li 2 S particles. Moreover, after 10 cycles, the capacity is stabilized around 500−550 mAh/g with a capacity decay rate of only ∼0.25% per cycle. The origin of the initial barrier is found to be the phase nucleation of polysulfides, but the amplitude of barrier is mainly due to two factors: (a) charge transfer directly between Li 2 S and electrolyte without polysulfide and (b) lithium-ion diffusion in Li 2 S. These results demonstrate a simple and scalable approach to utilizing Li 2 S as the cathode material for rechargeable lithium-ion batteries with high specific energy. ■ INTRODUCTIONRechargeable lithium-ion batteries have been widely used in portable electronics and are promising for applications in electric vehicles and smart grids. 1−4 However, due to limited capacity in both electrodes, the specific energy of Li-ion batteries needs to be improved significantly to fulfill the requirements in these applications. 5,6 Significant improvement has been achieved in the development of high-capacity materials to replace carbon-based anodes, such as silicon 7−12 and tin. 13 However, state-of-the-art cathode materials have a capacity less than one-half of the carbon anode. Accordingly, breakthroughs in cathodes are urgently needed to increase the specific energy of lithium-ion batteries. Current metal oxide and phosphate cathodes possess an intrinsic capacity limit of ∼300 mAh/g, with a potential of maximum 130% increase in the specific energy if all the capacity can be used. 14,15 In contrast, Li 2 S has a specific capacity of 1166 mAh/g, four times that of the limit in oxide/phosphate cathodes. 15,16 Considering pairing with Si anodes with 2000 mAh/g capacity, the specific energy of a Li 2 S-based lithium-ion battery could be 60% higher than the theoretical limit of metal oxide/phosphate counterparts ( Figure 1A, see Supporting Information for details) and three times that of the current LiCoO 2 /graphite system. Moreover, Li 2 S could be paired with a lithium-free anode, preventing safety concerns and low Coulomb efficiency of lithium metal in Li/S batteries. 17,18 The main hindrance for utilizing Li 2 S is that it is both electronically and ionically insulating. Therefore, Li 2 S was...

We use first principles calculations to study structural, vibrational and superconducting properties of H2S at pressures P ≥ 200 GPa. The inclusion of zero point energy leads to two different possible dissociations of H2S, namely 3H2S → 2H3S + S and 5H2S → 3H3S + HS2, where both H3S and HS2 are metallic. For H3S, we perform non-perturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction (λ ≈ 2.64 at 200 GPa) and Tc. Anharmonicity hardens H-S bond-stretching modes and softens H-S bond-bending modes. As a result, the electronphonon coupling is suppressed by 30% (λ ≈ 1.84 at 200 GPa). Moreover, while at the harmonic level Tc decreases with increasing pressure, the inclusion of anharmonicity leads to a Tc that is almost independent of pressure. High pressure hydrogen sulfide is a strongly anharmonic superconductor.Cuprates [1] have for many years held the world record for the highest superconducting critical temperature (T c = 133 K) [2]. However, despite almost 30 years of intensive research, the physical mechanism responsible for such a high T c is still elusive, although the general consensus is that it is highly non-conventional. The discovery by Drozdov et al.[3] of T c = 190 K in a diamond anvil cell loaded with hydrogen sulfide (H 2 S) and compressed to about 200 GPa breaks the cuprates record and overturns the conventional wisdom that such a high T c cannot be obtained via phonon-mediated pairing.The claim that hydrogen at high pressure could be superconducting is not new [4] and it was recently supported by first principles calculations based on the harmonic approximation applied to dense hydrogen [5][6][7][8] and several hydrides [9][10][11][12][13][14][15]. More recently, two theoretical papers predicted the occurrence of high T c superconductivity in high-pressure sulfur-hydrides [16,17]. However, as shown in Refs. [18,19], anharmonicity can be crucial in these systems. For example, in PdH, the electron-phonon coupling λ parameter is found to be 1.55 at the harmonic level, while a proper inclusion of anharmonic effects leads to λ = 0.40 [18], in better agreement with experiments. Thus, in hydrogen-based compounds, the phonon spectra are strongly affected by anharmonic effects.Several first principles calculations [16,17,20,26] suggested that decomposition of the H 2 S sample occurs within the diamond-anvil cell at high pressures. The high-T c superconducting material is therefore very unlikely to be H 2 S, while H 3 S is the obvious candidate for the H-rich decomposition product.Here we study the structural, vibrational and superconducting properties of H 2 S above 200 GPa, where the highest T c occurs. We show that the inclusion of zero point motion in the convex hull at 200 and 250 GPa stabilizes two metallic structures, H 3 S and HS 2 . Finally, we show that, contrary to suggestions in previous work [16,20], the harmonic approximation does not explain the measured T c ...

Rechargeable lithium-sulfur (Li-S) batteries hold great potential for high-performance energy storage systems because they have a high theoretical specific energy, low cost, and are eco-friendly. However, the structural and morphological changes during electrochemical reactions are still not well understood. In this Article, these changes in Li-S batteries are studied in operando by X-ray diffraction and transmission X-ray microscopy. We show recrystallization of sulfur by the end of the charge cycle is dependent on the preparation technique of the sulfur cathode. On the other hand, it was found that crystalline Li(2)S does not form at the end of discharge for all sulfur cathodes studied. Furthermore, during cycling the bulk of soluble polysulfides remains trapped within the cathode matrix. Our results differ from previous ex situ results. This highlights the importance of in operando studies and suggests possible strategies to improve cycle life.