Structural changes in Li 2 MnO 3 cathode material for rechargeable Li-ion batteries were investigated during the 1 st and 33 rd cycles by X-ray absorption spectroscopy. It is found that both the participation of oxygen anions in redox processes and Li + -H + exchange play an important role in the electrochemistry of Li 2 MnO 3 . During activation, oxygen removal from the material along with Li gives rise to the formation of a layered MnO 2 -type structure, while the presence of protons in the interslab region, as * To whom correspondence should be addressed † Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-

Although the lithium-metal anode has the advantages of both high gravimetric and volumetric capacities (3862 Ah kg −1 and 2085 Ah L −1 ) [13] and is already successfully used in primary batteries, it is still plagued by a series of issues that limit its successful operation in rechargeable applications, when organic solvent based electrolytes are used. [14] One of them is the nature of the lithium-metal dissolution and redeposition in the discharge and charge process together with the composition of the solid electrolyte interphase (SEI) [15,16] that is formed immediately after electrolyte addition and continues to form, grow and alter during cycling, [17] which limits the rechargeability in these battery systems and decreases their safety. [15,18,19] The SEI, though not being a homo geneous single phase, varies in composition and thickness and these differences lead to inhomogeneous and thus locally different current densities during the discharge and charge process, which can ultimately cause the formation of high surface area lithium (HSAL) during lithium deposition (charging) and hole/pit formation during dissolution (discharging). [20][21][22] In the worst case, the HSAL morphology takes the form of dendrites, i.e., small needle like lithium deposits that can grow through the separator from the anode towards the cathode. This process can lead to an internal short circuit of the cell resulting in local overheating and possibly cause a cell fire due to an increased reactivity with the electrolyte and the low melting point of lithium (180.54 °C). [23] Practical approaches to improve the rechargeable lithiummetal anode from the electrode material's point of view concentrate on either using coated lithium powder [24,25] or foil [26] and lithium with surface micropatterning. [27,28] The main underlying principle is increasing the specific surface area thus decreasing the effective current density and the resulting overpotential. However, the behavior of the lithium-metal electrode is quite complex and electrolyte-dependent [22,[29][30][31] and there is a need to identify the optimal conditions under which lithium-metal electrodes can cycle with both an increased reversibility and low overpotentials. [32] As-received lithium-metal foil contains several contaminants, [33] particularly on the surface. [34] In addition, even a new lithium foil that is considered to be smooth shows a non-negligible surface roughness that Lithium metal as an electrode material possesses a native surface film, which leads to a rough surface and this has a negative impact on the cycling behavior. A simple, fast, and reproducible technique is shown, which makes it possible to flatten and thin the native surface film of the lithium-metal anode. Atomic force microscopy and scanning electron microscopy images are presented to verify the success of the method and X-ray photoelectron spectroscopy measurements reveal that the chemical composition of the lithium surface is also changed. Furthermore, galvanostatic measurements indicate superior c...

No abstract

for anode materials to replace graphite [3] in lithium batteries. The application of thin lithium foils is considered to enable an increase in energy density by nearly a factor of 2 in case of solid-state lithium metal batteries (SSLMBs). [4] Besides LIB-derived cathode materials, lithium anodes are often considered for conversion-type cathodes enabling S 8 ||Li and O 2 ||Li batteries as well. [5] Nevertheless, lithium as anode material in rechargeable batteries has been extensively studied over four decades [6] and still no commercial application could be realized. [7] Major challenges concern the safety and uncontrolled lithium electrodeposition that often leads to the formation of high surface area lithium (HSAL), frequently called "dendrites." The high reactivity of pristine lithium and especially electrodeposited lithium leads to severe side reactions with electrolytes which result in the formation of a solid electrolyte interphase (SEI). [8] This SEI, however, is fractured during consecutive lithium electrodeposition and electrodissolution resulting in an inhomogeneous interphase on the lithium surface. These inhomogeneities can cause uncontrolled lithium electrodeposition and the formation of HSAL deposits. [9] Such deposits can occur in needle-like ( = dendritic) morphology and puncture the separator. After growing to the cathode, an internal short-circuit may be caused which dramatically increases the risk of a thermal runaway. To overcome the described issues, various approaches were developed based on electrolyte and lithium anode interphase engineering, [10] minimizing volume changes by using stable hosts, [11] and preventing dendrite formation and propagation by the use of solid-state electrolytes. [12] The stability of materials in contact with its environment is a well-known research area for corrosion science. Corrosion, in general, is defined as the chemical or electrochemical reaction between a material and its environment that results in a deterioration of the material and its properties. [13] As energy storage and conversion systems imply materials in a thermodynamically non-equilibrium state, corrosion-related processes are highly relevant for such systems. Figure 1a presents an overview of possible corrosion-related phenomena in a battery cell. Herein, we only discuss corrosion phenomena which occur in the presence of an electrolyte.At the positive electrode side, dissolution of Al, [14] which is typically used as a positive electrode current collector, and the cathode electrolyte interphase (CEI) [15] formation are phenomena related to corrosion in a battery cell (Figure 1b-d). One of the two processes which leads to dissolution of Al is Lithium metal is considered to be the most promising anode for the next generation of batteries if the issues related to safety and low coulombic efficiency can be overcome. It is known that the initial morphology of the lithium metal anode has a great influence on the cycling characteristics of a lithium metal battery (LMB). Lithium-powder-based el...