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While it is expected that inhomogeneity negatively affects battery performance, a quantitative understanding of the influence of inhomogeneity has remained elusive due to the difficulty of measurin...

Surface coating has been identified as an effective approach for enhancing the capacity retention of layered structure cathode. However, the underlying operating mechanism of such a thin coating layer, in terms of surface chemical functionality and capacity retention, remains unclear. In this work, we use aberration-corrected scanning transmission electron microscopy and high-efficiency spectroscopy to probe the delicate functioning mechanism of an Al2O3 coating layer on a Li1.2Ni0.2Mn0.6O2 cathode. We discovered that in terms of surface chemical function, the Al2O3 coating suppresses the side reaction between the cathode and the electrolyte during battery cycling. At the same time, the Al2O3 coating layer also eliminates the chemical reduction of Mn from the cathode particle surface, therefore preventing the dissolution of the reduced Mn into the electrolyte. In terms of structural stability, we found that the Al2O3 coating layer can mitigate the layer to spinel phase transformation, which otherwise will...

Immersion of a solid into liquid often leads to the modification of both the structure and chemistry of surface of the solid, which subsequently affects the chemical and physical properties of the system. For the case of the rechargeable lithium ion battery, consquence of such a surface modification is termed as solid electrolyte interphase (SEI) layer, which has been perceived to play a critical role in the stable operation of the batteries. However, the structure and chemical composition of the SEI layer and its spatial distribution and dependence on the battery operating conditions remain unclear. Using aberration-corrected scanning transmission electron microscopy, coupled with ultrahigh-sensitivity energy-dispersive X-ray spectroscopy, we probed the structure and chemistry of the SEI layer on several high-voltage cathodes. We show that layer-structured cathodes, when cycled at a high cut-off voltage, can form a Phosphorus-rich SEI layer on their surface, which is a direct evidence of Li-salt (LiPF6) ...

Capacity and voltage fading of layered structured cathode based on lithium transition-metal oxide is closely related to the lattice position and migration behavior of the transition-metal ions. However, it is scarcely clear about the behavior of each of these transition-metal ions in this category of cathode material. We report direct atomic resolution visualization of interatomic layer mixing of transition metals (Ni, Co, Mn) and lithium ions in layered structured oxide cathodes for lithium-ion batteries. Using chemical imaging with an aberration-corrected scanning transmission electron microscope (STEM) and density function theory calculations, we discovered that, in the layered cathodes, Mn and Co tend to reside almost exclusively at the lattice site of transition-metal (TM) layer in the structure or little interlayer mixing with Li. In contrast, Ni shows a high degree of interlayer mixing with Li. The fraction of Ni ions resides in the Li layer followed by a near linear dependence on total Ni concentr...

Capacity and voltage fading of Li2MnO3 is a major challenge for the application of this category of material, which is believed to be associated with the structural and chemical evolution of the materials. This paper reports the detailed structural and chemical evolutions of Li2MnO3 cathode captured by using aberration corrected scanning/transmission electron microscopy (S/TEM) after certain numbers of charge–discharge cycling of the batteries. It is found that structural degradation occurs from the very first cycle and is spatially initiated from the surface of the particle and propagates toward the inner bulk as the cyclic number increases, featuring the formation of the surface phase transformation layer and gradual thickening of this layer. The structure degradation is found to follow a sequential phase transformation: monoclinic C2/m → tetragonal I41 → cubic spinel, which is consistently supported by the decreasing lattice formation energy based on DFT calculations. For the first time, high spatial r...

We report systematic studies of the microstructural changes of uncoated and AlF3-coated Li-rich Mn-rich (LMR) cathode materials (Li1.2Ni0.15Co0.10Mn0.55O2) before and after cycling using a combination of aberration-corrected scanning/transmission electron microscopy (S/TEM) and electron energy loss spectroscopy (EELS). TEM coupled with EELS provides detailed information about the crystallographic and electronic structure changes that occur after cycling, thus revealing the fundamental improvement mechanism of surface coating. The results demonstrate that the surface coating reduces oxidation of the electrolyte at high voltage, suppressing the accumulation of a thick solid electrolyte interface (SEI) layer on electrode particle surface. Surface coating significantly enhances the stability of the surface structure and protects the electrode from severe etching/corrosion by the acidic species in the electrolyte, reducing the formation of etched surfaces and corrosion pits. Moreover, surface coating alleviate...

Copper substituted lithium iron phosphate, with the olivine structure, has been synthesized hydrothermally as dark-brown single crystals with 15% of the iron sites occupied by cupric ions. Single crystal X-ray diffraction and magnetic susceptibility,combined with Li solid state NMR studies performed on powder sample, showed that 5% of the lithium ions also reside on the iron site, giving the formula Li0.95[(Fe2+)0.70(Fe3+)0.10(Cu2+)0.15Li0.05)]PO4. The compound exhibits anisotropic magnetic susceptibility and orders antiferromagnetically at 48 K. The magnetic moments are aligned along [010] as in LiFePO4. The optical measurements indicate that the dark brown color originates from interionic d-d transitions. A part of the lithium ions can be cycled in an electrochemical cell, corresponding to the oxidation/reduction of the iron ions. An onset of a second electrochemical process is observed at about 4.1 V, tentatively attributed to the removal of Li ions from the Fe site. The first principles calculations i...

The layered oxide cathode material LiMO2, where M = Ni0.9−yMnyCo0.1 and 0.45 ≤ y ≤ 0.60, was synthesized by a coprecipitation method. X-ray diffraction analysis shows that the maximum manganese content in the stoichiometric material, i.e. with Li:M = 1, cannot exceed 50%; otherwise, a second phase is formed. Rietveld refinement reveals that increasing manganese content suppresses the disorder between the lithium and nickel ions. Magnetic measurements show that part of the Mn4+ ions in the manganese rich compounds is reduced to Mn3+; this results in a larger hysteresis loop due to the increased magnetic moment of the resulting ferrimagnetically ordered clusters. LiNi0.4Mn0.5Co0.1O2 and LiNi0.45Mn0.45Co0.1O2 show similar electrochemical capacities of around 180 mAh/g (between 2.5 and 4.6 V at 0.5 mA/cm2) for the first discharge. However, subsequent cycling of LiNi0.4Mn0.5Co0.1O2 results in faster capacity loss and poorer rate capability indicating that manganese rich compounds, with Li:M = 1:1, are probably...

The structural and physical properties of the low cobalt material, LiNi0.45Mn0.45Co0.1O2, have been investigated. Nearly stoichiometric LiNi0.45Mn0.45Co0.1O2 with the optimal electrochemical perfor...

Disordered nanocomposites comprised of polyethylene oxide and exfoliated MoS2 are cycled in the voltage range of 0.01−3.0 V. The nanocomposites showed a significant increase in reversible capacity and cycle stability when compared to exfoliated or bulk MoS2 and may be used as anode material in lithium ion batteries.