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A major challenge for Si negative electrodes in Li-ion batteries is how to accommodate the large lithiation-induced volume expansion and prevent electrode fragmentation, such that impressive Li storage capacity of Si can be exploited in practice. Electrochemically etched Si mesoporous electrodes have significant potential in this context. This research is focused on an optimum mesoporous Si electrode structure that shows a very high energy storage density, electrochemically cycling well without cracking or fragmentation. To explore the factors causing the superior performance, this study performed in-operandoneutron diffraction experiments on optimized electrode during lithiation-delithiation cycles in a simple in-situ electrochemical cell. It is shown that an unusual diffraction phenomenon arises from lithiation-induced expansion of Si leading to the development of mosaic structure in Si. This new phenomenon appears to arise from the increased contribution of kinematic diffraction from the lithiated Si, relative to the conventional dynamic diffraction. This is also supported by changes in diffraction intensities directly synchronizing with the volume changes in Si during lithiation-delithiation cycle. The in-operando experiments explain why the optimum mesoporous Si electrode possesses a high specific capacity without electrode fragmentation. The present findings can help to improve Si electrode designs for high energy density Li-ion batteries.

Li-Mg alloys are promising as positive electrodes (anodes) for Li-ion batteries due to the high Li storage capacity and the relatively lower volume change during the lithiation/delithiation process. They also present a unique opportunity to image the Li distribution through the electrode thickness at various delithiation states. In this work, spatial distributions of Li in electrochemically delithiated Li-Mg alloy electrodes have been quantitatively determined using neutron tomography. Specifically, the Li concentration profiles along thickness direction are determined. A rigorous analytical model to quantify the diffusion-controlled delithiation, accompanied by phase transition and boundary movement, has also been developed to explain the delithiation mechanism. The analytical modeling scheme successfully predicted the Li concentration profiles which agreed well with the experimental data. It is demonstrated that during discharge Li is removed by diffusion through the solid solution Li-Mg phases and this proceeds with β→α phase transition and the associated phase boundary movement through the thickness of the electrode. This is also accompanied by electrode thinning due to the change in molar volume during delithiation. Following the approaches developed here, one can develop a rigorous and quantitative understanding of electrochemical delithiation in electrodes of electrochemical cells, similar to that in the present Li-Mg electrodes.

The design and performance of a novel in-situ electrochemical cell that greatly facilitates the neutron diffraction study of complex phase transitions in small volume electrodes of Li-ion cells, is presented in this work. Diffraction patterns that are Rietveld-refinable could be obtained simultaneously for all the electrodes, which demonstrates that the cell is best suited to explore electrode phase transitions driven by the lithiation and delithiation processes. This has been facilitated by the use of single crystal (100) Si sheets as casing material and the planar cell configuration, giving improved signal-to-noise ratio relative to other casing materials. The in-situ cell has also been designed for easy assembly and to facilitate rapid experiments. The effectiveness of cell is demonstrated by tracking the neutron diffraction patterns during the charging of graphite/LiCoO2and graphite/LiMn2O4cells. It is shown that good quality neutron diffraction data can be obtained and that most of the finer details of the phase transitions, and the associated changes in crystallographic parameters in these electrodes, can be captured.

Electrochemical performance of Li(Mg) alloys as potential negative electrodes for Li-ion batteries has been investigated. Two Li(Mg) alloys with nominal compositions, Li-60 wt% Mg (Li7Mg3) and Li-30 wt% Mg (Li8Mg) were synthesized and electrodes were prepared by melting, rolling and annealing. Both the alloys showed a discharge plateau voltage of ∼2 V compared to the ∼2.5 V with pure Li anode with reference to MnO2as cathode. Detailed ex situ X-ray diffraction analysis revealed that during the discharge of Li(Mg) anodes, a gradual phase transformation, from the BCC Li(Mg) β-phase to the HCP Mg(Li) α-phase, occurred in the electrodes. An intriguing finding is that the Li-depleted Li(Mg) anode was largely intact as solid electrode even after delithiation. The Li(Mg) electrode showed some degree of reversibility (against LiCoO2), possibly by a combination of a small amount of Li alloying and/or deposition at the electrode-electrolyte interface during charging, a promising aspect for further investigation.