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Nanoscale Voltage Enhancement at Cathode Interfaces in Li-ion Batteries
Interfaces are ubiquitous in Li-ion battery electrodes, occurring across
compositional gradients, regions of multiphase intergrowths, and between
electrodes and solid electrolyte interphases or protective coatings. However,
the impact of these interfaces on Li energetics remains largely unknown. In
this work, we calculated Li intercalation-site energetics across cathode
interfaces and demonstrated the physics governing these energetics on both
sides of the interface. We studied the olivine/olivine-structured
LixFePO4/LixMPO4 (x=0 and 1, M=Co, Ti, Mn) and layered/layered-structured
LiNiO2/TiO2 interfaces to explore different material structures and transition
metal elements. We found that across an interface from a high- to low-voltage
material the Li voltage remains constant in the high-voltage material and
decays approximately linearly in the low-voltage region, approaching the Li
voltage of the low-voltage material. This effect ranges from 0.5-9nm depending
on the interfacial dipole screening. This effect provides a mechanism for a
high-voltage material at an interface to significantly enhance the Li
intercalation voltage in a low-voltage material over nanometer scale. We showed
that this voltage enhancement is governed by a combination of electron transfer
(from low- to high-voltage regions), strain and interfacial dipole screening.
We explored the implications of this voltage enhancement for a novel
heterostructured-cathode design and redox pseudocapacitors
Local probing of ionic diffusion by electrochemical strain microscopy: spatial resolution and signal formation mechanisms
Electrochemical insertion-deintercalation reactions are typically associated
with significant change of molar volume of the host compound. This strong
coupling between ionic currents and strains underpins image formation
mechanisms in electrochemical strain microscopy (ESM), and allows exploring the
tip-induced electrochemical processes locally. Here we analyze the signal
formation mechanism in ESM, and develop the analytical description of operation
in frequency and time domains. The ESM spectroscopic modes are compared to
classical electrochemical methods including potentiostatic and galvanostatic
intermittent titration (PITT and GITT), and electrochemical impedance
spectroscopy (EIS). This analysis illustrates the feasibility of spatially
resolved studies of Li-ion dynamics on the sub-10 nanometer level using
electromechanical detection.Comment: 49 pages, 17 figures, 4 tables, 3 appendices, to be submitted to J.
Appl. Phys
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