All-solid-state Li-ion batteries promise safer electrochemical energy storage with larger volumetric and gravimetric energy densities. A major concern is the limited electrochemical stability of solid electrolytes and related detrimental electrochemical reactions, especially because of our restricted understanding. Here we demonstrate for the argyrodite, garnet and NASICON type solid electrolytes, that the favourable decomposition pathway is indirect rather than direct, via (de)lithiated states of the solid electrolyte, into the thermodynamically stable decomposition products. The consequence is that the electrochemical stability window of the solid electrolyte is significantly larger than predicted for direct decomposition, rationalizing the observed stability window. The observed argyrodite metastable (de)lithiated solid electrolyte phases contribute to the (ir)reversible cycling capacity of all-solid-state batteries, in addition to the contribution of the decomposition products, comprehensively explaining solid electrolyte redox activity. The fundamental nature of the proposed mechanism suggests this is a key aspect for solid electrolytes in general, guiding interface and material design for all-solid-state batteries.3 All-solid-state-batteries (ASSBs) are attracting ever increasing attention due to their high intrinsic safety, achieved by replacing the flammable and reactive liquid electrolyte by a solid electrolyte 1 . In addition, a higher energy density in ASSBs may be achieved through; (a) bipolar stacking of the electrodes, which reduces the weight of the non-active battery parts and (b) by potentially enabling the use of a Li-metal anode, which possesses the maximum theoretical Li capacity and lowest electrochemical potential (3860 mAhg -1 and -3.04 V vs. SHE). First of all, the success of ASSBs relies on solid electrolytes with a high Li-ion conductivity 2-5 . A second prerequisite, is the electrochemical stability at the interfaces of the solid electrolyte with the electrode materials in the range of their working potentials. Any electrochemical decomposition of the solid electrolyte may lead to decomposition products with poor ionic conductivity that increase the internal battery resistance 2-4,6 . Third, ASSBs require mechanical stability as the changes in volume of the electrode materials upon (de)lithiation, as well as decomposition reactions at the electrode-electrolyte interface may lead to contact loss, also increasing the internal resistance and lowering the capacity 2-4 .

Zn-H 2 O fuel cells, [8] etc.), rechargeable aqueous Zn-ion batteries (ZIBs) with a mild electrolyte are particularly attractive as zinc is more compatible with water than alkaline metals, Zn-ions are divalent, and the production and recycling of these batteries is relatively simple. [9][10][11][12] A variety of manganese dioxide (MnO 2 ) polymorphs (α-, β-, γ-, δ-, and amorphous) have been investigated as cathodes materials for ZIBs. [10,11,[13][14][15][16][17][18][19] Several studies have demonstrated that the storage capacity of MnO 2 in a neutral or mildly acidic aqueous electrolyte is partly induced by a reversible Zn 2+ insertion/extraction reaction and partly by the reversible H + insertion/extraction. [10,11,19] This process is governed by the reversible phase transition of the MnO 2 polymorphs from a tunneled to layered structure, driven by the electrochemical reaction. During this phase transition, even though the crystalline structure is maintained, Mn 3+ in the tunnel chains are reduced to soluble Mn 2+ leading to a capacity fading, a short cycle life and low Coulombic efficiency of aqueous Zn/MnO 2 batteries. [17,20] To prevent this dissolution of Mn ions, alternative electrolytes were identified, for example, Zhang et al., found that by using a Zn(CF 3 SO 3 ) 2 aqueous electrolyte, Mn-ion dissolution was suppressed and a high capacity retention with a ZnMn 2 O 4 cathode was achieved. [21] Alternatively by using a MnSO 4 additive to a mild ZnSO 4 aqueous electrolyte, Pan et al. reported that the Mn 2+ dissolution in the aqueous Zn/MnO 2 electrolyte was inhibited. [11] Though research into MnO 2 cathodes for aqueous ZIBs has gained momentum, it is of great importance to develop alternative high capacity cathode materials for ZIBs that are stable in an aqueous electrolyte.Very recently, vanadium oxide and its related compounds have been reported as cathodes for ZIBs, showing higher energy densities and better capacity retention compared to MnO 2 cathodes. Senguttuvan et al. reported the reversible Zn 2+ insertion/extraction process in a V 2 O 5 cathode for a nonaqueous ZIBs. [22] Kundu et al. developed a highly stable vanadium bronze (Zn 0.25 V 2 O 5 ·nH 2 O) cathode for aqueous ZIBs, which displayed a high energy density and capacity retention through a reversible Zn 2+ (de)intercalation process. [12] In addition, LiV 3 O 8 , [23] H 2 V 3 O 8 , [24] and V 2 S [25] cathodes exhibit good capacity reversibility and power density for aqueous ZIBs. Although Oberholzer et al. [26] observed a proton intercalation Rechargeable aqueous zinc-ion batteries (ZIBs) are promising for cheap stationary energy storage. Challenges for Zn-ion insertion hosts are the large structural changes of the host structure upon Zn-ion insertion and the divalent Zn-ion transport, challenging cycle life and power density respectively. Here a new mechanism is demonstrated for the VO 2 cathode toward proton insertion accompanied by Zn-ion storage through the reversible deposition of Zn 4 (OH) 6 SO 4 ·5H 2 O on the cathode surface, sup...

Electrical mobility demands an increase of battery energy density beyond current lithium-ion technology. A crucial bottleneck is the development of safe and reversible lithium-metal anodes, which is challenged by short circuits caused by lithium-metal dendrites and a short cycle life owing to the reactivity with electrolytes. The evolution of the lithium-metal-film morphology is relatively poorly understood because it is difficult to monitor lithium, in particular during battery operation. Here we employ operando neutron depth profiling as a noninvasive and versatile technique, complementary to microscopic techniques, providing the spatial distribution/density of lithium during plating and stripping. The evolution of the lithium-metal-density-profile is shown to depend on the current density, electrolyte composition and cycling history, and allows monitoring the amount and distribution of inactive lithium over cycling. A small amount of reversible lithium uptake in the copper current collector during plating and stripping is revealed, providing insights towards improved lithium-metal anodes.

appear to be nicely spherical, whereas the surface layer has been oxidized. A native oxidation layer grows on the surface of individual Si nanoparticles when they are exposed to traces of air after synthesis. The presence of such limited pacifying oxide layer appeared of advantage for the further processing in air. The average thickness of the oxidation layer is around 1.2 nm, and it is amorphous when observed by X-ray diffraction (XRD) (Figure 1 c), i.e., only peaks corresponding to crystalline Si are visible. For a particle with a size of 20 nm Si in diameter and 1.2 nm outer layer of SiO 2 , the volume percentage of SiO 2 is 28.8%. Raman spectra (Figure 1 d) on the sample report that both crystalline and amorphous Si exist and the amount of amorphous Si is signifi cant (c-Si:a-Si = 0.39:0.61; quantitative analysis in the Supporting Information). To determine the amount of oxygen in the sample, thermogravimetric analysis (TGA) is carried out by heating the Si NP sample under a mixture of O 2 /Ar gas and fully oxidizing Si into SiO 2 . The result indicates that the amount of Si accounts for 69.0 wt% of the sample ( Figure S2, Supporting Information), i.e., Si:SiO 2 = 0.83:0.17 in mole. Meanwhile, the volume fraction from the estimated mass ratio above is 28.2% and is in good quantitative agreement with the one estimated from TEM.Galvanostatic tests on the Si NP electrode are performed using different dis-/charge currents between applied potentials of 0.01 and 2.8 V. In this paper, all specifi c currents applied are calculated with respect to the mass of Si. De-/sodiation capacities of Si stated in this paper are the capacities after subtracting the capacity of the super P carbon black ( Figure S3, Supporting Information), and excluding inactive SiO x inside the sample.Figure 2 a demonstrates an initial sodiation capacity of 1027 mAh g −1 for Si at 20 mA g −1 , which is higher than the theoretical capacity (954 mAh g −1 for NaSi). A large part of this initial capacity is attributed to the irreversible formation of a solid electrolyte interface (SEI) layer on the surface of Si in combination with some decomposition of electrolyte, and possibly also the irreversible formation of sodium silicate from reaction with SiO 2 . The subsequent Na ion extraction process achieves a capacity up to 270 mAh g −1 , indicating that a signifi cant Na fraction is stored reversibly, next to the large irreversible part. For the subsequent few cycles the sodiation capacity decreases from above 410 mAh g −1 to around 300 mAh g −1 but the desodiation capacity is relatively stable around 260 mAh g −1 . The Coulombic effi ciency grows gradually to >90%, after which the de-/sodiation capacity becomes relatively stable. After 100 cycles the reversible capacity retention reaches 248 mAh g −1 , which is 92% of the fi rst desodiation capacity; and the Coulombic efficiency declines slowly to 87% in this cycle test. Additionally, Figure 1 e shows that after charge/discharge for 100 cycles Si particles in this electrode got fractured into small g...

Phosphorus and tin phosphide based materials that are extensively researched as the anode for Na‐ion batteries mostly involve complexly synthesized and sophisticated nanocomposites limiting their commercial viability. This work reports a Sn4P3‐P (Sn:P = 1:3) @graphene nanocomposite synthesized with a novel and facile mechanochemical method, which exhibits unrivalled high‐rate capacity retentions of >550 and 371 mA h g−1 at 1 and 2 A g−1, respectively, over 1000 cycles and achieves excellent rate capability (>815, ≈585 and ≈315 mA h g−1 at 0.1, 2, and 10 A g−1, respectively).