Lithium metal batteries have been considerably limited by the problems of uncontrolled dendritic lithium formation and the highly reactive nature of lithium with electrolytes. Herein, we have developed functional porous bilayer composite separators by simply blade-coating polyacrylamide-grafted graphene oxide molecular brushes onto commercial polypropylene separators. Our functional porous bilayer composite separators integrate the lithiophilic feature of hairy polyacrylamide chains and fast electrolyte diffusion pathways with the excellent mechanical strength of graphene oxide nanosheets and thus enable molecular-level homogeneous and fast lithium ionic flux on the surfaces of electrodes. As a result, dendrite-free uniform lithium deposition with a high Coulombic efficiency (98%) and ultralong-term reversible lithium plating/stripping (over 2600 h) at a high current density (2 mA cm−2) are achieved for lithium metal anodes. Remarkably, lithium metal anodes with an unprecedented stability of more than 1900 h cycling at an ultrahigh current density of 20 mA cm−2 are demonstrated.

The nickel-rich layered oxide materials have been selected as promising cathode materials for the next generation lithium ion batteries because of their large capacity and comparably high operating voltage. However, at high voltage (beyond 4.30 V vs Li/Li), the members of this family are all suffering from a rapid capacity decay, which was commonly concerned with crystal lattice distortion and related cation disordering. In this work, the quasi-spherical Ni-rich layered LiNiCoMnO (QS-NMC622) material was successfully synthesized through the carbonate co-precipitation method. A coupled measurement, which is a combination of potentiostatic intermittent titration technique (PITT) and in situ X-ray diffraction (XRD), was deployed to simultaneously capture the structural changes and lithium ion diffusion coefficient of QS-NMC622 material during the first cycle. With help of in situ XRD patterns and high-resolution transmission electron microscope (HR-TEM) images, a defective spinel framework of Fd3̅m space group was detected along with a rapid decreasing lattice-parameter c and lattice distortion at deep delithiated state, which causes poor kinetics related to lithium ion mobility. The new-born framework seems to transform and remain as full spinel structure in the parent phase to the end of charge/discharge with high voltage, which could deteriorate both the surface and body structure stability during the subsequent cycles. This established coupled in situ measurement could be applied to simultaneously investigate the structure transformation and kinetics of cathode materials during charge/discharge.

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LiNiCoMnO positive electrode materials of lithium ion battery can release a discharge capacity larger than 200 mAh/g at high potential (>4.30 V). However, its inevitable capacity fading, which is greatly related to the structural evolution, reduces the cycling performance. The origin of this capacity fading is investigated by coupled in situ XRD-PITT-EIS. A new phase of NiMnO is discovered on the surface of the LiNiCoMnO upon charging to high voltage, which blocks Li diffusion pathways. Theoretical calculations predict the formation of cubic NiMnO. Moreover, corrosion, cracks, and microstress appear to increase the difficulty of Li transportation, which are attributed to the protection degradation of the interfacial film on the positive electrode material at high voltage. After 50 electrochemical cycles, the increase in degree of crystal defects by low-angle grain boundary, evidenced through HR-TEM, leads to poor Li kinetics, which in turn causes capacity loss. The in situ XRD-PITT-EIS technique can bring multiperspective insights into fading mechanism of the high-voltage positive electrode materials and provide a solution to control or suppress the problem on the basis of structural, kinetic, and electrochemical interfacial understandings.

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In Li1.2Ni0.12Co0.15Mn0.53O2 oxide, four lines of lattice fringe from O3 rhombohedral phase are squeezed into two lines from spinel cubic phase at 4.70 V charged state under the effect of microstress.

The electrolyte additive plays an important role in determining the crucial properties of batteries such as cycling stability and safety. Compared to material development, research on electrolyte and interphase is still in the early stage for sodium ion batteries (SIBs). Herein, for the first time, succinic anhydride (SA) is investigated as a synergistic filming additive to fluoroethylene carbonate (FEC), and the lifespan of the dual‐additive Na/Na0.6Li0.15Ni0.15Mn0.55Cu0.15O2 (NLNMC) cell is significantly improved, maintaining capacity retention of 87.2% over 400 cycles at 1 C rate. For comparison, the batteries with only one of the two additives or without any additive show much inferior electrochemical performance. After the addition of SA, the interphase layer on the surface of cycled NLNMC material becomes uniform and stable, which contains more oxygen‐rich organic species and less NaF. Additionally, the addition of SA also has an impact on the interphase layer in the sodium anode part as indicated by electrochemical impedance spectroscopy (EIS) and energy dispersive spectrometer (EDS) results. Moreover, the online differential electrochemical mass spectrometry (OEMS) tests show the dual‐additive cell has less CO2 generation during the initial two cycles compared to that with only FECs which demonstrates another advantage of SA for practical application.

The design of the anode material is considered the core subject in manufacturing a battery system, among which, Li is a commonly used, ideal anode material. Normally, the electrolyte is decomposed on the Li metal anode to form a solid electrolyte interphase (SEI) layer. [4] Owing to this layer, the contact between the electrolyte and Li metal decreases, causing the batteries to be recyclable and efficient. During batteries cycling, however, a large volume expansion of the Li anode may lead to a rupture of the SEI layer, exposing fresh Li that could react with the electrolyte. The rupture of SEI therefore leads to a low coulombic efficiency (CE) and poor cycling stability. [5,6] Generally, the decomposition of Li salts in electrolytes results in the SEI layer that comprises inorganic compounds, while the electrolyte solvents are composed mainly of organic components. [7,8] Some researchers have reported that increasing the content of inorganics in the SEI layer is beneficial for improving its stability and mechanical strength. [9][10][11] Different strategies for modulating the Li anode interface were investigated, including the use of electrolyte additives and artificial SEI layers. [12][13][14][15][16][17][18] Among them, regulating the composition of the SEI layers by changing the solvation structure of Li + using electrolyte additives seemed simpler. [19] During the transfer of Li + through the SEI layer, Li + desolvation causes coordination anions and solvents to decompose and spontaneously form the SEI layer. [20][21][22][23] Unfortunately, most electrolyte additives themselves participate in the formation of the SEI layer. [24][25][26] Moreover, the effect of the additives gradually disappears owing to their continuous decomposition. By contrast, the artificial SEI layers, where extra protective layers are grafted directly onto the surface of Li metal, will not decompose significantly as the batteries cycle. Instead, the artificial SEI layer inhibits the growth of dendrite because of its specific mechanical properties. [27][28][29][30] In addition, it helps disperse Li + after passing through the membrane, homogenize the flux of Li + , and reduce the formation of dendrites. [31][32][33][34] Due to the remarkable effect of artificial layers, previous researches on the nature of the protective layers themselves are intense. [35][36][37][38][39] However, it should be noted that artificial layer has always a certain lifetime, and we suppose Li metal has been attracting considerable attention as the most promising anode material for application in next-generation Li rechargeable batteries. However, the instability of the formed solid electrolyte interphase (SEI) in the Li metal anode leads to a low coulombic efficiency (CE). Here, two kinds of synthesized polymer materials with different molecular configurations (chain and cross-linked), which are grafted as skins on Cu foils (current collectors), are reported. The interaction between the polymers and electrolyte solvent reduces the amount of the solvent used in...