Article Cite This: Chem. Mater. 2018, 30, 3656−3667
pubs.acs.org/cm
Origin of High Capacity and Poor Cycling Stability of Li-Rich Layered Oxides: A Long-Duration in Situ Synchrotron Powder Diffraction Study Karin Kleiner,*,†,‡ Benjamin Strehle,† Annabelle R. Baker,‡ Sarah J. Day,‡ Chiu C. Tang,‡ Irmgard Buchberger,† Frederick-Francois Chesneau,§ Hubert A. Gasteiger,† and Michele Piana†
Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 1, 2018 at 20:18:46 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
Chair of Technical Electrochemistry, Department of Chemistry and Catalysis Research Center, Technical University of Munich, D-85748 Garching, Germany ‡ Harwell Science and Innovation Campus, Diamond Light Source, Didcot, Oxfordshire OX11 0DE, U.K. § Badische Anilin- & Soda-Fabrik Societas Europaea (BASF SE, GCN/EE - M311), D-67056 Ludwigshafen, Germany S Supporting Information *
ABSTRACT: High-energy Li1.17Ni0.19Co0.10Mn0.54O2 (HENCM) is a lithium-rich layered oxide with alternating Liand transition-metal (TM) layers in which excess lithium ions replace transition metals in the host structure. HE-NCM offers a capacity roughly 50 mAh g−1 higher compared to that of conventional layered oxides but suffers from capacity loss and voltage fade upon cycling. Differential capacity plots (taken over 100 cycles) show that the origin of the fading phenomenon is a bulk issue rather than a surface degradation. Although previous studies indicate only minor changes in the bulk material, long duration in situ synchrotron X-ray powder diffraction measurements, in combination with difference Fourier analysis of the data, revealed an irreversible transition-metal motion within the host structure. The extensive work provides new insights into the fading mechanism of the material.
■
INTRODUCTION
still not satisfying, or apparently successful modifications were only compared to poor reference samples.6,7 Less attention has been paid to understanding the fading mechanism of the material, which could help to find a way for improving its cycling stability. Until now, the common understanding is that oxygen release from the host structure is the main reason for capacity drop and voltage fade.8−10 However, more recent online electrochemical mass spectrometry and transmission electron microscopy (TEM) studies suggest that an irreversible lattice-oxygen oxidation is limited to near-surface parts of the particles and is associated with the formation of a spinel or rock-salt structure, causing high overpotentials (in charge and discharge).11−15 Irreversible changes observed over extended cycling data show an overpotential increase only for the discharge. Furthermore, according to differential capacity plots, redox processes appear, change, and shift with an increasing number of cycles. Both of these observations cannot be explained by surface effects because such processes would change the overpotentials of the charge and discharge to the same extent and would not lead to new peaks in the differential capacity plots.4,6,16 Therefore, the
Since the introduction of lithium-ion batteries in 1991 by Sony, their global market reached several billion US dollars and is still increasing. The main driving force of this trend is the renaissance of electromobility. Climatic changes (caused by the combustion of fossil fuels), the limited availability of nonrenewable energy sources, and a growing environmental awareness are arguments leading to this trend. However, these arguments will not lead to a large-scale commercialization of battery electric vehicles (BEVs) as long as their price is higher and their range is shorter in comparison to the current fossilfuel-powered vehicles.1,2 Both factors are related to the energy density of lithium-ion batteries, which is limited up to now by the cathode active materials.2,3 Lithium-rich layered oxides offer a reversible discharge capacity of 250 mAh g−1 at a relatively high mean discharge voltage of 3.5 V (vs graphite) and would therefore fulfill the energy density requirements of the automotive industry.1,2 However, the material suffers from a poor capacity retention and voltage fade upon cycling, which currently hinders its market penetration.4,5 Many efforts seeking to improve these issues have been put into doping and/or surface modifications.6,7 In many cases, this led to better cycling stabilities, with the drawback of a lower capacity, but the capacity retention was © 2018 American Chemical Society
Received: January 12, 2018 Revised: May 3, 2018 Published: May 3, 2018 3656
DOI: 10.1021/acs.chemmater.8b00163 Chem. Mater. 2018, 30, 3656−3667
Article
Chemistry of Materials
during the first charge was studied using a fast position-sensitive detector (PSD) at the beamline.20 The energy of the X-ray beam was tuned to ∼15 keV, and the calibrated wavelength was λ= 0.826117(10) Å. A battery cell was mounted onto an xyz-stage that was adjusted to the center of the diffraction instrument. In situ long-term studies (105 cycles) from two identical battery cells were performed using SXPD on the recently commissioned long duration experiment (LDE) facility at the beamline. The detailed technical description of the LDE instrument is given by Murray et al.21 Using a higher energy beam of ∼25 keV (λ ≈ 0.496 Å), SXPD patterns were taken every week upon long-term cycling. Data were collected with a 2D Pixium area detector at a detector distance of ∼0.49 m. CeO2 (NIST Standard Reference Material 674b, United States) was measured before every HE-NCM pattern to refine the wavelength and the detector distance each time and evaluate the instrumental reflection broadening. During data collection, both cells were held at open circuit voltage (OCV), one in the charged and one in the discharged state. The charge/discharge during OCV alternated every week to assess the reproducibility of the experiment and have powder diffraction patterns in both the charged and the discharged state at the same cycle. The exposure time during the powder diffraction measurements was 5 min. The data were reduced with the software package DAWN22,23 and refined with the software package Fullprof (2θ range: 0−40°).24 Due to preferential orientations of aluminum (pouch foil, current collector) and lithium (counter electrode), these phases were included in the refinements using the Le Bail method (only profile fitting). When these reflections are excluded, partially overlapping HE-NCM reflections would also be excluded, which means a loss of information. As the HE-NCM reflections exhibited a mismatch in intensities and an unusual broadening (especially observed for the charged states), difference Fourier (DF) analysis with the software packages WinGX and VESTA was used to evaluate and confirm disorder within the crystallographic structure.25,26 Reflection broadening due to increased microstrain during charge of HE-NCM was further analyzed with Rietveld refinement to enable the determination of Li/transition-metal (TM) disorder parameters in the charged material. A detailed description of the refinement can be found in the Supporting Information, Sections S4 and S5. Inductively coupled plasma optical emission spectroscopy (ICPOES) was performed at Mikroanalytisches Labor Pascher (Remagen, Germany). Prior to the measurements, the materials were cycled in pouch cells (2 and 30 cycles, respectively). The pouch cells were disassembled in an argon-filled glovebox and the cathode materials were washed with dimethyl carbonate. In addition, pristine HE-NCM, also coated on an aluminum current collector, was analyzed. The materials were removed from the current collector and the obtained powder was dissolved with an acid digestion. ICP-OES was performed using a Thermo Scientific iCAP 6500 duo instrument. For each measurement, 5−10 mg of the powder was taken and for every sample, 2 measurements were performed.
electrochemical performance suggests structural changes (e.g., disorder) as the origin of the observed drop in the cycling performance, which are more suitably studied by methods sensitive to the bulk structure (e.g., powder diffraction).17 Although very pronounced structural changes have been observed, e.g. with TEM13,18 in surface near parts of the particles, these changes are not observed when investigating the bulk structure and are therefore not relevant to the conclusions made in the work. Nevertheless, the incidence of transitionmetal migration during cycling is supported by theoretical calculations.19 At best, the idea of the fading mechanism is currently too simplified, and a deeper understanding is necessary. Furthermore, the question of why lithium-rich materials offer a capacity 50 mAh g−1 higher compared to that of structurally related layered oxides still remains unanswered. In the present study, new insights into the origin of the poor cycling stability and the high capacity of lithium-rich layered oxides are obtained from long-duration synchrotron X-ray powder diffraction (SXPD) measurements. Using pouch cells suitable for in situ SXPD, the electrochemistry and structural behavior of HE-NCM was studied over long-term cycling, performed for the first time at the long duration experiment facility I11 at the Diamond Light Source, United Kingdom. Although there is no evidence for changes in the bulk structure geometry of the unit cell, it is possible to determine and quantify transition-metal (TM) migration upon cycling by detailed reflection profile analysis and difference-Fourier mapping. The relationship of reversible and irreversible transition-metal disorder with the strain in the material can provide guidance to improve the cycling stability of lithium-rich cathodes or to synthesize application-oriented materials in the future.
■
MATERIALS AND METHODS
High-energy Li1.17Ni0.19Co0.10Mn0.54O2 (HE-NCM) electrodes (92.5 wt % BASF SE HE-NCM, 2% Timcal SFG6L graphite, 2% Timcal Super C65, 3.5% Solef PVDF, ≈6.3 mgHE‑NCM cm−2) were cycled in pouch cells (cathode: 30 × 30 mm2) versus lithium (33 × 33 mm2, 0.45 mm thick, 99.9% Rockwood Lithium, United States). To avoid short circuits due to lithium dendrite formation upon cycling, four glass-fiber separators (36 × 36 mm2, glass microfiber filter 691, VWR, Germany) were used, and 1.5 mL LP57 (1 M LiPF6 in EC:EMC 3:7 by weight,
