Elsevier

Applied Energy

Volume 231, 1 December 2018, Pages 446-455
Applied Energy

Identifying degradation mechanisms in lithium-ion batteries with coating defects at the cathode

https://doi.org/10.1016/j.apenergy.2018.09.073Get rights and content

Highlights

  • The impact of electrode coating defects on cell performance is evaluated.

  • Interactions between cathode and anode cause degradation in cells with defects.

  • Computational analysis validates findings from chemical characterization.

  • Electrodes with certain coating defects can be repurposed for less demanding uses.

  • Repurposing electrodes will reduce scrap rates and lower manufacturing cost.

Abstract

Understanding the effect of electrode manufacturing defects on lithium-ion battery (LIB) performance is key to reducing the scrap rate and cost during cell manufacturing. In this regard, it is necessary to quantify the impact of various defects that are generated during the electrode coating process. To this end, we have tested large-format 0.5 Ah LiNi0.5Mn0.3Co0.2O2/graphite pouch cells with defects intentionally introduced into the cathode coating. Different types of coating defects were tested including agglomerates, pinholes, and non-uniform coating. Electrodes with larger non-coated surface had greater capacity fade than baseline electrodes, while pinholes and agglomerates did not affect performance adversely. Post cycle analysis of electrodes showed that the anode facing the defective region in the cathode was clearly impacted by the defect. Further characterization using Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction provided evidence for a proposed mechanism for material degradation related to the most detrimental type of coating defect.

Introduction

Lithium-ion batteries (LIBs) have been successfully commercialized in portable electronic devices [1], [2]. However, batteries for electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs) [3] have stricter requirements like lighter weight, longer range, improved safety, and longer cycle life [4]. EV batteries need to be cost-effective as well. The latest EV cell cost target from the US Department of Energy (DOE) is $80/kWh with a useable energy density of 750 Wh/L and a peak power density of 1500 W/L by 2020. This is a very aggressive target, since the current cells for electric vehicles cost around $245/kWh and have an energy density around 285 Wh/L.[5] The cost of expensive metals like cobalt, nickel, and lithium is a major component of the final price of LIBs [6]. On this subject, there is growing research aimed towards lowering the cost of materials in LIBs [7]. However, any change in the battery materials has to undergo a rigorous testing phase to address strict safety requirements and to exhibit equivalent or improved battery performance [8]. Therefore, new material technologies require long periods for implementation. To tackle the immediate need for cost reduction, there is a growing demand to reduce the manufacturing costs to offset high material costs [9].

There are several methods to reduce electrode manufacturing cost through advanced material processing and material handling technologies. These include adapting cheaper water-based solvents, developing solvent-free coating [10], or implementing spray/electrostatic coating methods [11]. However, changing processing technologies at an existing battery manufacturing plant requires a high capital cost. Another method to reduce cost without large capital investment is to improve quality control practices to reduce scrap rates. The cost of raw materials in the electrode is high, and coating defects are one of the primary sources of waste in battery manufacturing. The current quality control process involves discarding defective coatings regardless of the type of defect and using only ideal electrode coatings, potentially contributing to excessive waste. The quality of the electrode coating depends on uniform thickness, porosity [12], material distribution (areal weight), and adhesion to the current collector [13]. Any inhomogeneity of these properties results in defects [14] that lead to local aging of the electrode with loss in capacity and cycle life [15].

In this study, we evaluate the effect of electrode inhomogeneities on the electrochemical behavior of lithium-ion batteries. We analyzed the electrochemical properties of three types of coating defects in cathodes: (a) pinholes, (b) agglomerates, and (c) line defects. In our previous study, we used coin cells for analysis which showed that defects in electrode coating significantly impacted electrochemical performance [16]. However, the study was influenced by cell-to-cell variations, and the fraction of defective area relative to the total electrode area was large, which exaggerated the influence of the defect. To overcome these limitations, we continued our studies using large-format multilayer pouch cells, which closely resembled industrial battery manufacturing conditions. To our knowledge this is the first study to evaluate the impact of different types of coating defects on the electrochemical performance of lithium-ion batteries under conditions that replicate state-of-the-art electrode coating and cell manufacturing. Cell testing was performed using a rigorous protocol to accelerate the degradation process. Further, chemical characterization using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) was performed on the harvested electrodes to determine the mechanism of degradation and the extent to which it influences electrochemical properties. As in our previous study [16], the electrode defects were specifically generated in a controlled manner to standardize the experiment. By better understanding how cells with coating defects behave, we are able to determine if these coatings are suitable for other applications. For example, electrodes with some defects could possibly be used in low-power or low-energy applications like grid-storage or backup power storage devices instead of becoming waste.

Section snippets

Materials and electrode fabrication

The electrodes were fabricated at the U.S. Department of Energy (DOE) Battery Manufacturing R&D Facility at Oak Ridge National Laboratory. Anode and cathode slurries were prepared by dispersing the active material, binder, and conductive additives in N-Methyl-2-pyrrolidone with a planetary mixer (Ross PDM-1/2). The cathode consisted of LiNi0.5Mn0.3Co0.2O2 powder (NMC532, Toda America Inc., 90 wt%), polyvinylidene fluoride (PVDF, Solvay 5130, 5 wt%), and carbon black (Denka, 5 wt%). The anode

Results and discussion

Defects generated during coating can be broadly classified into four categories as outlined in our previous paper [16]. They are (1) agglomerates (blisters), (2) pinholes (divots), (3) line defects and (4) metal particle contamination. Of these defects, metal particle contamination is the most detrimental, but it can be managed by maintaining a clean and dust-free coating environment. The schematic in Fig. 1 shows the three kinds of defects that are primarily generated during slurry processing

Conclusion

This study reveals the effect of cathode coating defects on the electrochemical performance of lithium-ion cells. We analyzed four types of defects that commonly occur during the coating process. Electrochemical cycling of cells made with these defects showed that pinholes and agglomerates did not lead to significant loss in capacity. However, cells with non-uniform coatings in the form of line defects showed more severe capacity fade. Post-cycle characterization of the electrodes with one 3 mm

Acknowledgement

This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) (Deputy Director: David Howell) Applied Battery Research subprogram (Program Manager: Peter Faguy). X-ray Diffraction was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Part of

Author contributions

D.W. and D.M. pursued the idea and D.W. and L.D. led this research. L.D. performed electrochemical testing and material characterization, analyzed the data, and wrote the manuscript. R.E.R. performed Raman spectroscopy and contributed to analysis and discussion. H.M. collected X-ray photoelectron spectroscopy data, and contributed to analysis and discussion. Y.S. made the NMC electrode, and assisted in electrochemical testing. S.K. performed numerical modeling and contributed to the discussion.

References (28)

  • K. Edström et al.

    A new look at the solid electrolyte interphase on graphite anodes in Li-ion batteries

    J Power Sources

    (2006)
  • H. Zheng et al.

    Correlation between dissolution behavior and electrochemical cycling performance for LiNi1/3Co1/3Mn1/3O2-based cells

    J Power Sources

    (2012)
  • T. Takamura et al.

    Charge/discharge efficiency improvement by the incorporation of conductive carbons in the carbon anode of Li-ion batteries

    J Power Sources

    (2000)
  • D. Larcher et al.

    Towards greener and more sustainable batteries for electrical energy storage

    Nat Chem

    (2015)
  • Cited by (42)

    View all citing articles on Scopus

    This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

    View full text