High Power Batteries probed by Neutron Scattering
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Rechargeable batteries are currently powering a large range of applications, including power and emergency storage systems, HEVs and EVs, and various portable electronic devices. Increasing the use of batteries as a sustainable energy storage technology for the future requires further efforts in studies of the battery chemistry to achieve breakthroughs in battery performances. Two different types of rechargeable batteries were in focus for the present study: nickel metal hydride- and Li-ion batteries. Advanced characterization techniques that allow for understanding the deepest details of lithium/hydrogen insertion and removal mechanisms to/from the novel materials are crucial for selecting the best compound for demanding applications. This work aims on the use of neutron scattering methods - diffraction and imaging - for investigating the mechanism of transformations in high performance rechargeable batteries. In situ neutron diffraction was employed to investigate the structural evolution of the electrode materials in an ICR 10440 commercial cylindrical lithium-ion battery, which has a discharge capacity of 360 mAh. A three-phase mixture of Li(Ni,Mn,Co)O2, LiCoO2 and LiMn2O4 was identified as the active material of the cathode, with graphite acting as the anode material. The study revealed that the graphite anode underwent structural changes to form a series of insertion-type lithiated derivatives, with up to 12.7% volume expansion for the Li-saturated compound LiC6. The charge-discharge behaviour was more complex for the cathode. Here, the charge process was associated with partial lithium depletion from the initially Li-saturated compounds, leading to volume shrinkage for Li(Ni,Mn,Co)O2, in contrast to (Ni,Mn)-free LiCoO2. Electrochemical discharge experiments performed under a fast regime (2 C) at 5, 25 and 45 ºC revealed that the discharge capacity followed the trend of an increased diffusion rate of Li+ ions in the electrolyte and Li atoms in both electrodes, being highest for 45 ºC. At the lowest tested temperature (5 ºC), a rapid drop in the discharge capacity took place using the same kinetic regime. In-operando neutron radiography provided unprecedented insight into the macrostructural deformations and Li redistribution along the battery depending on the state of charge and current density. Deviations from a homogeneous Li distribution behaviour have been found in the radial and axial directions of the battery with increased current density. Radiography further reveals the expulsion and circulation of the electrolyte between the layers and absorption of the electrolyte at high current rates. Furthermore, tomography reveals the structural deformations along the axial and radial direction of the battery. Despite remarkable progress, lithium ion batteries still need higher energy density and better cycle life for consumer electronics, electric drive vehicles and large-scale renewable energy storage applications. Magnesium-Silicon alloys has been explored as anode materials for high energy lithium ion batteries; however, attaining long cycle life remains a significant challenge due to material degradation during cycling and an unstable solid-electrolyte interphase. This study explores the possibilities of nanostructuring the Mg-Si alloy by different synthesis techniques: hydrogen driven route, casting and rapid solidification. Stoichiometric Mg2Si (Mg67Si33) and its Si-rich eutectic (Mg47Si53) compositions were synthesized and characterised as an anode for lithium ion battery. The electrochemical tests indicate that the Si rich eutectic electrode shows a higher charge-discharge capacity than the stoichiometric Mg2Si. Rapidly solidified Mg2Si and eutectic alloys showed the highest initial discharge capacities of 989 mAh/g and 1283 mAh/g, respectively, superior to the alloys prepared by hydrogen desorption-recombination process and casting. A complex multistep reaction mechanism is suggested for lithiation in the alloy: (a) lithium insertion into the Mg2Si, followed by (b) the dissociation of Mg and alloying of silicon and lithium and (c) consecutive lithiation of Mg in the follow-up electrochemical step. The effect of electrolyte additives Fluoroethylene carbonate (FEC) and vinyl carbonate (VC) was investigated and it enhanced the cycling capacity of the electrodes because of the formation of the stable SEI layer. La2MgNi9-related alloys are superior metal hydride battery anodes as compared to the commercial AB5 alloys. Nd-substituted La2-yNdyMgNi9 intermetallics are of particular interest because of increased diffusion rate of hydrogen and thus improved performance at high discharge currents. The structural evolution of LaNdMgNi9 intermetallic as anode for the nickel metal hydride (Ni-MH) battery was probed in operando by neutron diffraction. The alloy exhibited a single pressure plateau in the PCT isotherm at 293 K with a maximum hydrogen storage capacity close to 13 H/f.u. corresponding to a high electrochemical discharge capacity of 373 mAh/g, which is 20% higher than the AB5 type alloys. During the electrochemical cycling, formation of an extended α- solid solution spanning from LaNdMgNi9D0.8 to LaNdMgNi9D1.6 was observed, whereas the homogeneity range for the β- deuteride occurred between LaNdMgNi9D8.2 and LaNdMgNi9D9.8. Nd substitution contributes to the high-rate dischargeability, while maintaining a good cyclic stability. Electrochemical Impedance Spectroscopy (EIS) characterization showed a decreased hydrogen transport rate during the long-term cycling, which indicated a corresponding slowing down of the electrochemical processes at the surface of the metal hydride anode. In summary, this study explores the possibility of neutron scattering techniques to perform insitu time resolved studies on complex lithium ion and metal hydride battery systems by giving insight into the mechanism and dynamics of the phase-structural transformations. Novel anode materials - Mg-Si and LaNdMgNi9 alloys have been developed for Li ion and metal hydride batteries.