Distinguishing Li<sup>+</sup>Charge Transfer Kinetics at NCA/Electrolyte and Graphite/Electrolyte Interfaces, and NCA/Electrolyte and LFP/Electrolyte Interfaces in Li-Ion Cells

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190

Understanding the factors limiting Li + charge transfer kinetics in Li-ion batteries is essential in improving the rate performance, especially at lower temperatures. The Li + charge transfer process involved in the lithium intercalation of graphite anode includes the step of de-solvation of the solvated Li + in the liquid electrolyte and the step of transport of Li + in the preformed solid electrolyte interphase (SEI) on electrodes until the Li + accepts an electron at the electrode and becomes a Li in the electrode. Whether the de-solvation process or the Li + transport through the SEI is a limiting step depends on the nature of the interphases at the electrode and electrolyte interfaces. Several examples involving the electrode materials such as graphite, lithium titanate (LTO), lithium iron phosphate (LFP), lithium nickel cobalt aluminum oxide (NCA) and solid Li + conductor such as lithium lanthanum titanate or Li-Al-Ti-phosphate are reviewed and discussed to clarify the conditions at which either the de-solvation or the transport of Li + in SEI is dominating and how the electrolyte components affect the activation energy of Li + charge transfer kinetics. How the electrolyte additives impact the Li + charge transfer kinetics at both the anode and the cathode has been examined at the same time in 3-electrode full cells. The resulting impact on Li + charge transfer resistance, R ct , and activation energy, E a , at both electrodes are reported and discussed. To improve the power performance of Li-ion batteries, it is important to understand the factors that limit the Li + charge transfer kinetics. Li-ion batteries comprised of a graphite anode and a lithium cobalt oxide cathode in an electrolyte consisting of 1 M LiPF 6 in ethylene carbonate (EC)-dimethyl carbonate (DMC)-diethyl carbonate (DEC) carbonate solvent mixture could not deliver their room temperature capacity at a rate of C/2 at −30 and −40• C. 1 When DEC was replaced by a linear ester solvent, such as ethyl acetate (EA) or methyl butyrate (MB), the Li-ion batteries at −30 and −40• C could deliver over 80% of their room temperature capacity at the same rate.2 When the LiPF 6 salt is replaced by lithium bis(oxalato)borate (LiBOB) in EC-ethyl methyl carbonate (EMC) (1:1 wt ratio) carbonate solvent mixture, the impedance of the graphite-electrolyte interface measured using graphite/Li half cells in the electrolyte with LiBOB is three times that in the electrolyte with LiPF 6 .3 These examples illustrate that the electrolyte components play crucial roles in affecting Li + charge transfer kinetics in Li-ion batteries.The Li + charge transfer process starts from the solvated Li + in the electrolyte to the reception of an electron (e − ) from the electrode and becomes Li (i.e., Li x C in graphitic anodes). This involves the desolvation step of Li + before entering into a layer of solid electrolyte interphases, or SEI, that is often referred to that on the anode such as carbonaceous materials, and the diffusion step of Li + through the SEI, which is pre-form...

Section: Discussionmentioning

confidence: 99%

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confidence: 99%

Factors Limiting Li⁺Charge Transfer Kinetics in Li-Ion Batteries

Jow

Delp

Allen

et al. 2018

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302

190

“…Here, the lithium concentration in the solid (10700 mole/m 3 ) is calculated for an LNMO particle at 50% SOC (≡ 70 mAh/g LNMO = 252 As/g LNMO ), using a bulk density of 4.4 g/cm 3 (252 As/g ·4.4 g/cm 3 Based on this result, the solid state diffusion inside the active material is negligible, i.e., we can omit the Warburg element in series to the charge transfer resistance in the equivalent circuit model in Figure 1, but must place a Warburg element in series to the separator resistance to capture the effect of liquid diffusion.…”

Section: Discussionmentioning

confidence: 99%

“…In general, the measured cell and/or electrode impedances are usually fitted with a serial connection of an ohmic resistor (R), with a parallel circuit of a resistor and a capacitor (C), commonly referred to as R/C element and often also modified to an R/Q element (Q representing a constant-phase element), as well as with a Warburg element (W). [1][2][3][4][5] Recently, more elaborate equivalentcircuits using a transmission line model are getting more and more attention. [6][7][8] In order to independently obtain the impedance of anode and cathode, there are two possible options: i) the assembly of symmetric cells as shown by Chen et al 9 or Petibon et al, 10 where coin cells out of two anodes (impedance of the negative electrode) or two cathodes (impedance of the positive electrode) are assembled in a glove box or dry-room from two (aged) full-cells at a specified state-of-charge (SOC); ii) the use of three-electrode setups consisting of a working electrode (WE), a counter electrode (CE) and a reference electrode (RE), which allows to individually determine the impedance of the anode and the cathode of a lithium ion battery full-cell.…”

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confidence: 99%

An Analysis Protocol for Three-Electrode Li-Ion Battery Impedance Spectra: Part I. Analysis of a High-Voltage Positive Electrode

Landesfeind

Pritzl

Gasteiger

2017

134

199

A key for the interpretation of porous lithium ion battery electrode impedance spectra is a meaningful and physically motivated equivalent-circuit model. In this work we present a novel approach, utilizing a general transmission line equivalent-circuit model to exemplarily analyze the impedance of a porous high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO) cathode. It is based on a LNMO/graphite full-cell setup equipped with a gold wire micro-reference electrode (GWRE) to obtain impedance spectra in both, non-blocking conditions at a potential of 4.4 V cell voltage and in blocking configuration achieved at 4.9 V cell voltage. A simultaneous fitting of both spectra enables the deconvolution of physical effects to quantify over the course of 85 cycles at 40 • C: a) the true charge transfer resistance (R CT ), b) the pore resistance (R Pore ), and c) the contact resistance (R Cont. ). We demonstrate that the charge transfer resistance would be overestimated significantly, if the spectra are fitted with a conventionally used simplified R/Q equivalent-circuit compared to our full transmission line analysis. Advanced analysis techniques for lithium ion batteries are a key requirement to deconvolute the complex interplay between the aging mechanisms occurring at the anode and the cathode. In principle, this can be accomplished by electrochemical impedance spectroscopy (EIS), if the individual contributions of anode and cathode to the overall cell impedance can be determined, and if this EIS response can be fitted unambiguously to physically motivated equivalent-circuit models. In general, the measured cell and/or electrode impedances are usually fitted with a serial connection of an ohmic resistor (R), with a parallel circuit of a resistor and a capacitor (C), commonly referred to as R/C element and often also modified to an R/Q element (Q representing a constant-phase element), as well as with a Warburg element (W). [1][2][3][4][5] Recently, more elaborate equivalentcircuits using a transmission line model are getting more and more attention. [6][7][8] In order to independently obtain the impedance of anode and cathode, there are two possible options: i) the assembly of symmetric cells as shown by Chen et al. 9 or Petibon et al., 10 where coin cells out of two anodes (impedance of the negative electrode) or two cathodes (impedance of the positive electrode) are assembled in a glove box or dry-room from two (aged) full-cells at a specified state-of-charge (SOC); ii) the use of three-electrode setups consisting of a working electrode (WE), a counter electrode (CE) and a reference electrode (RE), which allows to individually determine the impedance of the anode and the cathode of a lithium ion battery full-cell. The latter is a more convenient approach, as individual impedance spectra can be recorded continuously during battery cycling, so that anode and cathode impedance can be monitored during cycle-life studies on a fullcell instead of obtaining only one set of anode and cathode impedance spectra after disassembly of a full-cell...

“…Both the desolvation process and the SEI formed at the electrode and electrolyte interface could limit the charge transfer kinetics. [16][17][18] Recent studies indicate that the specific SEI formed on the electrode could exert more influence on the charge transfer kinetics. 18 The electrons pass through from the Li metal to the anode through their direct contact point due to the existing potential difference.…”

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confidence: 99%

Investigation of Pre-lithiation in Graphite and Hard-Carbon Anodes Using Different Lithium Source Structures

Shellikeri

Watson

Adams

et al. 2017

119

113

Energy storage devices like batteries and supercapacitors, have become indispensable in portable devices and electric vehicles. But, the carbon anodes used in these devices, exhibit a significant loss in their initial capacity (graphite: 6.6% and hard carbon: 29.8%) due to the consumption of Li ions from electrolyte during solid electrolyte interface layer formation and other non-reversible reactions, and if unaddressed, can potentially diminish this advantage. Pre-lithiation treatment of carbon anodes can mitigate this loss, apart from improving the cell working potential, hence enhancing cell energy density and cyclability. We investigate in this report a direct-contact lithiation process in graphite and hard carbon anodes using four distinct lithium source structures. The results show that the type of Li-source structure employed for anode pre-lithiation can have a major impact on the electrode physical properties, pre-lithiation times and the pre-lithiation anode potentials achieved, potentially effecting the SEI properties, cycle life, and the electrode processing costs. for their reversible characteristic, where a guest ion (e.g. Li-ion) can be reversibly intercalated and de-intercalated into the host carbon material. On the other hand, the host carbon anode materials when in contact with organic electrolytes (Li salt in organic solvents), form an electronically passivating solid electrolyte interface (SEI) layer 3 on their surface by the reduction of electrolyte. This ensures the longterm stability of the device by passivating the carbon anode surface by preventing further reduction of the electrolyte on the anode surface. This SEI layer is ionically conductive enough to provide Li-ion pathways passing through it for further intercalation into the bulk of anode after de-solvating them, therefore preventing the co-intercalation of solvent molecules into the carbon structure. Although, the formation of SEI layer is necessary and beneficial, nevertheless, it consumes the Li-ions from the electrolyte, depleting it of charge carriers. This is the initial irreversible cycle loss 4 and if not compensated for, then it can drastically affect the efficiency and cyclability of the energy storage device. 5 The initial cycle loss can be mitigated by supplying additional Li-ions from Li reservoirs and this process is generally known as pre-lithiation. Apart from mitigating the initial cycle loss, pre-lithiation also improves the working potential of the device by providing a lower redox potential to the carbon anode, hence raising the overall cell potential, which further results into higher cell energy density, cycle stability and reduced electrode resistance. 2,6 Moreover, due to the pre-lithiation process, the anion-cation concentration in the electrolyte remains mostly stable during cycling, hence maintaining a steady electrolyte conductivity, which is crucial for cell efficiency and cyclability.Pre-lithiation can be achieved through several methods, 7 including electrochemically driven lithiation, 8 lithiation usi...