Search

Voltage decay,i.e., the voltage decrease during electrochemical cycling, has been a decade-long challenge for lithium-ion batteries. This issue not only leads to a substantial loss of energy density, but also raises challenges for the battery management system, hindering the commercial application of high capacity lithium-rich oxide. Here, we show that through a combination of electrochemical conditioning and thermal treatment, Li2MnO3, the parent compound of lithium-rich oxide, which typically displays severe voltage and capacity decay, could be converted into a new phase that essentially suppresses the voltage decay with improved capacity retention and rate performance. By combining atomic-sensitive nuclear magnetic resonance, differential electrochemical mass spectrometry and synchrotron-based resonant inelastic X-ray scattering, we disclose that treatment triggers the formation of three-coexisting phases,i.e., the lithium-rich layered, spinel and defect spinel phases, which enables improved reversibility of the oxygen redox activity and enhanced manganese redox reactions in the initial cycle. Our findings suggest the key role of the local structure in the voltage decay problem and provide insights for material optimizations towards lithium- and manganese-rich cathodes without the voltage decay.

As a parent compound of Li-rich electrodes, Li2MnO3 exhibits high capacity during the initial charge; however, it suffers notoriously low Coulombic efficiency due to oxygen and surface activities. Here, we successfully optimize the oxygen activities toward reversible oxygen redox reactions by intentionally introducing protons into lithium octahedral vacancies in the Li2MnO3 system with its original structural integrity maintained. Combining structural probes, theoretical calculations, and resonant inelastic X-ray scattering results, a moderate coupling between the introduced protons and lattice oxygen at the oxidized state is revealed, which stabilizes the oxygen activities during charging. Such a coupling leads to an unprecedented initial Coulombic efficiency (99.2%) with a greatly improved discharge capacity of 302 mAh g-1 in the protonated Li2MnO3 electrodes. These findings directly demonstrate an effective concept for controlling oxygen activities in Li-rich systems, which is critical for developing high-energy cathodes in batteries.

Rechargeable lithium metal batteries based on organic electrolytes face challenges of both lithium metal cycling stability and the associated safety issues. Herein, we demonstrate an in situ formed polymer gel electrolyte which enables dendrite-free lithium metal cycling. Moreover, the gel electrolyte goes through further polymerization at elevated temperatures and loses its ionic conductivity, effectively shutting down the battery. When lithium iodide (LiI) is dissolved in vinylene carbonate (VC), LiI induces the polymerization of VC to form poly(vinylene carbonate) (polyVC). The electrolyte then transforms into a polymer gel electrolyte containing VC as the solvent and LiI as the salt. At room temperature, the gel electrolyte enables dendrite-free lithium metal cycling at current densities as high as 5 mA cm-2 for 500 cycles. Furthermore, a Li/Li4Ti5O12 (LTO) cell retains 50% of the initial capacity at the 700th cycle. When the cell is heated to 80 °C, the ionic resistance of the electrolyte increases by a factor of 103, resulting in the shutdown of the cell due to the complete polymerization of VC. The approach of using in situ polymerization to enable stable lithium cycling and to serve as a thermally triggered shutdown mechanism provides a new pathway for fabricating safer lithium metal batteries.

Density functional theory is coupled with experiment to explore the ability of substitutional doping to improve oxygen stability in Li-rich layered oxide bulk.

Anisotropic disorder along the c-axis results from static disorder.

Anisotropic disorder along the c-axis results from static disorder.

Spontaneous and anisotropic surface segregation of W cations in LiMn1.5Ni0.5O4 particles can alter the Wulff shape and improve surface stability, thereby significantly improving the electrochemical performance. An Auger electron nanoprobe was employed to identify the anisotropic surface segregation, whereby W cations prefer to segregate to {110} surface facets to decrease its relative surface energy according to Gibbs adsorption theory and subsequently increase its surface area according to Wulff theory. Consequently, the rate performance is improved (e.g., by ∼5-fold at a high rate of 25C) because the {110} facets have more open channels for fast lithium ion diffusion. Furthermore, X-ray photoelectron spectroscopy (XPS) depth profiling suggested that the surface segregation and partial reduction of W cation inhibit the formation of Mn3+ on surfaces to improve cycling stability via enhancing the cathode electrolyte interphase (CEI) stability at high charging voltages. This is the first report of using anisotropic surface segregation to thermodynamically control the particle morphology as well as enhancing CEI stability as a facile, and potentially general, method to significantly improve the electrochemical performance of battery electrodes. Combining neutron diffraction, an Auger electron nanoprobe, XPS, and other characterizations, we depict the underlying mechanisms of improved ionic transport and CEI stability in high-voltage LiMn1.5Ni0.5O4 spinel materials.