Voltage fade is a major problem in battery applications for high-energy lithium- and manganese-rich (LMR) layered materials. As a result of the complexity of the LMR structure, the voltage fade mechanism is not well understood. Here we conduct both in situ and ex situ studies on a typical LMR material (Li1.2Ni0.15Co0.1Mn0.55O2) during charge–discharge cycling, using multi-length-scale X-ray spectroscopic and three-dimensional electron microscopic imaging techniques. Through probing from the surface to the bulk, and from individual to whole ensembles of particles, we show that the average valence state of each type of transition metal cation is continuously reduced, which is attributed to oxygen release from the LMR material. Such reductions activate the lower-voltage Mn3+/Mn4+ and Co2+/Co3+ redox couples in addition to the original redox couples including Ni2+/Ni3+, Ni3+/Ni4+ and O2−/O−, directly leading to the voltage fade. We also show that the oxygen release causes microstructural defects such as the formation of large pores within particles, which also contributes to the voltage fade. Surface coating and modification methods are suggested to be effective in suppressing the voltage fade through reducing the oxygen release. Voltage decay is a major problem in applications of high-energy Li- and Mn-rich layer-structured battery materials. Here, the authors report the evolution of redox couples as the origin of the voltage decay and discuss strategies to suppress the problem.

Evolution of redox couples in Li- and Mn-rich cathode materials and mitigation of voltage fade by reducing oxygen release

LiCoO2 is a dominant cathode material for lithium-ion (Li-ion) batteries due to its high volumetric energy density, which could potentially be further improved by charging to high voltages. However, practical adoption of high-voltage charging is hindered by LiCoO2’s structural instability at the deeply delithiated state and the associated safety concerns. Here, we achieve stable cycling of LiCoO2 at 4.6 V (versus Li/Li+) through trace Ti–Mg–Al co-doping. Using state-of-the-art synchrotron X-ray imaging and spectroscopic techniques, we report the incorporation of Mg and Al into the LiCoO2 lattice, which inhibits the undesired phase transition at voltages above 4.5 V. We also show that, even in trace amounts, Ti segregates significantly at grain boundaries and on the surface, modifying the microstructure of the particles while stabilizing the surface oxygen at high voltages. These dopants contribute through different mechanisms and synergistically promote the cycle stability of LiCoO2 at 4.6 V. LiCoO2 is a widely used cathode material in Li-ion batteries for applications such as portable electronics. Here, the authors report multiple-element doping to enable stable cycling of LiCoO2 at high voltages that are not yet accessible with commercial Li-ion batteries.

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Trace doping of multiple elements enables stable battery cycling of LiCoO2 at 4.6 V

Rechargeable battery technologies have ignited major breakthroughs in contemporary society, including but not limited to revolutions in transportation, electronics, and grid energy storage The remarkable development of rechargeable batteries is largely attributed to in-depth efforts to improve battery electrode and electrolyte materials There are, however, still intimidating challenges of lower cost, longer cycle and calendar life, higher energy density, and better safety for large scale energy storage and vehicular applications Further progress with rechargeable batteries may require new chemistries (lithium ion batteries and beyond) and better understanding of materials electrochemistry in the various battery technologies In the past decade, advancement of battery materials has been complemented by new analytical techniques that are capable of probing battery chemistries at various length and time scales Synchrotron X-ray techniques stand out as one of the most effective methods that allow for near

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Synchrotron X-ray Analytical Techniques for Studying Materials Electrochemistry in Rechargeable Batteries

By breaking through the energy density limits step-by-step, the use of lithium cobalt oxide-based Li-ion batteries (LCO-based LIBs) has led to the unprecedented success of consumer electronics over the past 27 years. Recently, strong demands for the quick renewal of the properties of electronic products every so often have resulted in smarter, larger screened, more lightweight devices with longer standby times that have pushed the energy density of LCO-based LIBs nearly to their limit. As a result, with the aim of achieving a higher energy density and lifting the upper cut-off voltage of LCO above 4.45 V (vs. Li/Li+), the development of LCO-based all-solid-state lithium batteries (ASSLBs) with a Li metal anode and LCO-based full cells with high-performance anodes have become urgent scientific and technological requirements. This review summarizes the key challenges of synthesizing LCO-based LBs with a higher energy density from the perspectives of structure and interface stability, and gives an account of effective modification strategies in view of the electrodes, liquid electrolytes, binders, separators, solid electrolytes and LCO-based full cells. The improvement mechanisms of these modification strategies and the controversy over them are also analyzed critically. Moreover, some perspectives regarding the remaining challenges for LCO-based LBs towards a higher energy density and possible future research focuses are also presented.

Reviving lithium cobalt oxide-based lithium secondary batteries-toward a higher energy density

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research

For designing new battery systems with higher energy density and longer cycle life, it is important to understand the degradation mechanism of the electrode material, especially at the individual particle level. Using in situ transmission X-ray microscopy (TXM) coupled to a pouch cell setup, the inhomogeneous Li distribution as well as the formation, population, and evolution of inactive domains in a single LiCoO2 particle were visualized as it was cycled for many times. It is found that the percentage of the particle that fully recovered to the pristine state is strongly related to the cycling rate. Interestingly, we also observed the evolution of the inactive region within the particle during long-term cycling. The relationship between morphological degradation and chemical inhomogeneity, including the formation of unanticipated Co metal phase, is also observed. Our work highlights the capability of in situ TXM for studying the degradation mechanism of materials in LIBs.

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In situ Visualization of State-of-Charge Heterogeneity within a LiCoO2 Particle that Evolves upon Cycling at Different Rates

https://escholarship.org/content/qt4fk7w4d3/qt4fk7w4d3.pdf?t=p9ajm2

Charge Heterogeneity and Surface Chemistry in Polycrystalline Cathode Materials

Structural integrity—Searching the key factor to suppress the voltage fade of Li-rich layered cathode materials through 3D X-ray imaging and spectroscopy techniques

The SPEAR3 timing system includes a 1.28 MHz beam revolution clock and a 10 Hz pulse train for charge injection into the main storage ring. In the past the 10 Hz pulse train was used to study transient injected beam dynamics using turn-by-turn BPMs and visible-light synchrotron radiation diagnostics. For this work the timing distribution system was configured to extract solitary pulses from the 10 Hz pulse train to synchronize bunch-bybunch data acquisition with visible light diagnostics during transient beam events. The synchronous measurements are used to study controlled grow/damp and drive/damp beam physics.

Time-Synchronized Beam Diagnostics at SPEAR3

Recently, Quick-Scanning Extended X-ray Absorption Fine Structure Spectroscopy (QEXAFS) has become an important tool for in-situ characterization of materials and measurement of associated electronic structure for where samples have a short life due to X-ray damage. In this case, the time response of the ionization chamber affects the measurement resolution and therefore overall performance of the QEXAFS system. Common parallel-plate ionization chambers have a step-response rise time of about 0.1 sec, which does not meet the requirements of QEXAFS. To speed up the response, we constructed a gridded ionization chamber with variable bias voltage and optional background gas (N2 or He, respectively). To characterize the system we used a high-frequency beam chopper upstream of the ionization chamber and a highspeed, low-noise preamplifier to measure the step response of the chamber as a function of bias voltage and background gas conditions.

Zhihong Sun

Papers

In situ Visualization of State-of-Charge Heterogeneity within a LiCoO2 Particle that Evolves upon Cycling at Different Rates

Charge Heterogeneity and Surface Chemistry in Polycrystalline Cathode Materials

Structural integrity—Searching the key factor to suppress the voltage fade of Li-rich layered cathode materials through 3D X-ray imaging and spectroscopy techniques

Time-Synchronized Beam Diagnostics at SPEAR3

Time Response of a Gridded X-ray Beam Ionization Chamber