Mechanistic Study of Electrolyte Additives to Stabilize High-Voltage

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Mechanistic Study of Electrolyte Additives to Stabilize HighVoltage Cathode-Electrolyte Interface in Lithium-Ion Batteries Han Gao, Filippo Maglia, Peter Lamp, Khalil Amine, and Zonghai Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15395 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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ACS Applied Materials & Interfaces

Mechanistic Study of Electrolyte Additives to Stabilize HighVoltage Cathode-Electrolyte Interface in Lithium-Ion Batteries

Han Gao1, Filippo Maglia2, Peter Lamp2, Khalil Amine1*, and Zonghai Chen1*

1. Chemical Science and Engineering Division, Argonne National Laboratory, Lemont, IL, USA, 60439 2. BMW Group, Munich, Germany, 80788

Corresponding Authors *K. Amine. E-mail: [email protected]. *Z. Chen. E-mail: [email protected].

1 ACS Paragon Plus Environment

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Abstract Current developments of electrolyte additives to stabilize electrode-electrolyte interface in Lithium-ion batteries highly rely on a trial-and-error search, which involves repetitive testing and intensive amount of resources. The lack of understandings on the fundamental protection mechanisms of the additives significantly increases the difficulty for the transformational development of new additives. In this study, we investigated two types of individual protection routes to build a robust cathode-electrolyte interphase at high potentials: (i) a direct reduction in the catalytic decomposition of the electrolyte solvent; and (ii) formation of a “corrosion inhibitor film” that prevents severely attack and passivation from protons that generated from the solvent oxidation, even the decomposition of solvent cannot be mitigated. Effect of two exemplary electrolyte additives, lithium difluoro(oxalato)borate (LiDFOB) and 3-hexylthiophene (3HT), on LiNi0.6Mn0.2Co0.2O2 (NMC 622) cathode were investigated to validate our hypothesis. It is demonstrated that understandings of both electrolyte additives and solvent are essential and careful balance between the cathode protection mechanism of additives and their side effects is critical to obtain optimum results. More importantly, this study opens up new directions of rational design of functional electrolyte additives for the next generation high-energy density lithium-ion chemistries.

Key words: lithium-ion batteries; high voltage; electrode-electrolyte interface; electrolyte additives; protection mechanisms, LiDFOB; 3HT


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Introduction Research on rechargeable Lithium-ion batteries has aimed to provide high-performance energy storage solutions for the fast growing areas of consumer and industrial applications.1-5 One major target for the next-generation Lithium-ion battery is to reach even wider operational voltage windows, which involves the utilization of high-voltage cathodes.6 For instance, the working potential of the layered lithium Ni-Mn-Co oxides (NMC) can be pushed to ca. 4.6 V,7-8 the Li-rich xLi2MnO3·(1-x)LiMO2 (M = Ni, Co, Mn) has a working potential of over 4.5 V,9-10 and the spinel LiNi0.5Mn1.5O4 belong to the 5 V class.11-13 However, cycling of these cathodes with high working potentials always leads to fast capacity fades and low columbic efficiencies (CE) in conventional electrolytes. Aggressive oxidization of the electrolyte solvent is one well-known cause of the poor cycling performance of high-voltage cathodes. Even within the commonly considered stable working potential window of the electrolyte (usually measured using inert electrodes such as Pt), the slow oxidation reaction of the carbonate solvents will always occur due to the much more reactive nature of the delithiated cathodes compared to inert electrodes.14-15 This leads to a continued growth of surface layers on the cathode and an increase in the electrode impedance over cycling. It is generally accepted that the cycling performance, CE, and rate capability of the cathode are highly dependent on this surface layer. In addition, the oxidation of solvents also results in as highly reactive protons and unwanted gases.1, 15-16 Incorporation of electrolyte additives is an efficient and economical way to suppress the detrimental reactions. Various additives have been reported to stabilize the cathode-electrolyte interface at high potentials in the past decade.17-20 It is commonly believed that these additives have lower oxidation potentials than the carbonate solvents, such that the oxidation of the additive can first



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produce a more robust cathode-electrolyte interphase covering the cathode surface. This reduces the direct contact between electrolyte and electrode, thus, the oxidative decomposition of the electrolytes and any parasitic reactions can be suppressed. However, search for these additives heavily rely on a trial-and-error process, which involve repetitive testing and long-term cycling of the cells. Generally, the effectiveness of the additives are often concentration-dependent, which means it is also critical and necessary to control the amount of additives added to the electrolyte via engineering optimization. These further require an intensive amount of resources and are extremely time-consuming. One major reason behind the increasing difficulty for the transformational development of new high-voltage additives is the lack of fundamental understanding on the protection mechanisms of the additives. In this study, we will emphasize on the mechanistic study of electrolyte additives to stabilize high-voltage cathode-electrolyte interface by suppressing the impact of the parasitic reactions. Two different types of protection routes that can generate a robust cathode-electrolyte interphase are proposed here. To validate our proposed mechanisms, effect of two well-known additives from two different families, boron-based lithium difluoro(oxalato)borate (LiDFOB) and conjugated polymer-based 3-hexylthiophene (3HT), are studied. The purpose of this paper is to provide a new learning pathway towards fundamental understandings on the protection mechanisms of additives rather than focusing on any engineering optimization around a specific chemistry. We will show our proposed mechanisms not only optimize the cycling stability and impedance of the cathodes, but also help to select a proper design guideline for the future development of new high-voltage additives.


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Experimental Section Preparation of electrolytes The control electrolyte used was 1 M LiPF6 in ethylene carbonate (EC)-ethyl methyl carbonate (EMC) (3:7 by weight, BASF LP-57). Different amount of LiDFOB (battery grade, Central glasses) or 3HT (≥99%, Aldrich) was added to the baseline LP-57 for different electrolyte formulations. Table 1 shows the concentrations used for the two additives as well as their chemical structures. The LiDFOB-containing electrolytes were prepared in 2%, 1%, and 0.5% while lower concentrations of 0.5%, 0.25%, and 0.05% were chosen for 3HT (all wt. %). The rational for a low concentration of 3HT will be detailed in a latter session. Preparation of electrodes and cells LiNi0.6Mn0.2Co0.2O2 (NMC 622, Ecopro Co.) working electrodes were prepared by laminating a slurry onto Al foil substrates using a doctor blade. The NMC 622 powders was specifically prepared for research purposes with no surface coatings or doping. The slurry was composed of 91.5 wt. % NMC, 4.4 wt. % conductive agent (C45 carbon black), and 4.1 wt. % polyvinylidene fluoride (PVdF) binder. After laminating, the electrodes were first dried at 75 ºC in air overnight and subsequently calendered and punched (14 mm in diameter). All punched electrodes were vacuum-dried at ca. 110 ºC overnight prior to cell assembly. The NMC mass loading on each electrode was ca. 20 mg. A lower NMC mass loading of 5 mg was also prepared to study the oxidation potential of the additives in the differential capacity plots. NMC half-cells with varying electrolytes were then constructed in a CR2032 coin cell configuration using Celgard 2325 separator and Li metal reference/counter electrodes in a glovebox filled with Ar.



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Electrochemical characterizations Charge-discharge tests were conducted on the CR2032-type half-cells on a MACCOR series 4000 battery tester at room temperature (ca. 25 ºC). A home-build high-precision leakage current measuring system (based on Keithley 2401 source meters) was used to obtain the leakage current.7 The working electrodes were held at each specific potential for 40 hours to reach an equilibrium state at 30 ºC. Electrochemical impedance spectroscopy (EIS) spectra were recorded on a Solartron 1400A frequency response analyzer interfaced with the Solartron 1470E from 100 kHz to 0.01 Hz with 10 mV amplitude. Battery surrogates with the same cell cable configuration and connection fixture with a four-terminal connection scheme were used to maximize the accuracy of EIS measurements.

Results and Discussions Interfacial reactivity between NMC and conventional electrolyte Before going into the protection mechanisms of high-voltage electrolyte additives, halfcells containing baseline electrolyte were first examined by using NMC 622 as a model cathode material. Figure 1a shows the 4th cycle specific discharge capacity as well as the 1st cycle CE values of the baseline half-cells without any electrolyte additives under a C-rate of 0.1 (1 C = 185 mAg-1). An almost-linear increase in capacity can be obtained by pushing the upper cut-off potential from 4.4 to 4.6 V vs. Li/Li+. Both the enhancement in specific capacity as well as the average working voltage are very desirable for the next-generation high energy density Lithiumion cells. However, the increase of upper cut-off potential also caused a faster kinetics of the parasitic reactions, as evidenced by the decreasing trend of CE. One most important parasitic reaction is the oxidation of the electrolyte solvent at high potentials.


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The reactivity of the baseline electrolyte with NMC 622 was further investigated by extracting the static leakage current as a function of potential. This measures the rate of electron transfer reaction (i.e. oxidation of electrolyte solvent) through the solid-electrolyte interface. Figure S1a shows one example of the high-precision leakage current data for three NMC 622/Li cells with the baseline electrolyte held at 4.5 V vs. Li/Li+. All three cells showed good reproducibility and their leakage current decayed to less than 50 nAmg-1 at the end of potentiostatic hold, reaching their steady state. Electrical signal noise (nA level) was smoothed and the resulting curves were averaged for the pseudo-linear region of the relaxation curves as shown in Figure S1b. An exponential decay function was used to extract the static leakage current values (y0) from the current relaxation curves of cells. This fitting was performed at different potentials and the resulting values are plotted in Figure 1b (each data point represents an average of at least three cells). In general, the static leakage current values increased exponentially with potential, as predicted by the Tafel equation. This clearly shows the severe electrochemical oxidation of electrolyte components at the surface of the NMC 622 electrode at high potentials. This oxidation of solvent certainly had detrimental effects on the cycling performance of NMC 622. Figures 1c shows a trend of lower capacity retention with a trend of higher upper cutoff potential when the baseline cells were cycled at 4.4, 4.5, and 4.6 V vs. Li/Li+ (C-rate = 0.1). Further investigation on these baseline cells focused on their impedance response to quantify the increase in electrode surface film thickening and electrode “corrosion” due to solvent oxidation. Figures 1d show the Nyquist plots of the baseline cell at 4.4 V vs. Li/Li+ obtained during cycling between 2.8 to 4.4 V vs. Li/Li+ under a constant current of C/10. The impedance spectra showed two overlapping semi-circles and a “tail”. The semicircle at the high-mid frequency window



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display the charging/discharging of the electrochemical double-layer and the contribution of total surface film resistances while the other one at the mid-low frequency represents the rate of the charge-transfer reaction. The “tail” at the low frequency region is affected by the Li ion diffusion, which was not included in the data fitting in this study. An equivalent circuit is shown in Figure 1d, where Rs is the equivalent series resistance, Rfilm is surface film resistance, RCT is charge-transfer resistance, Qfilm and QCT are the constant phase elements to take into account of the non-ideal capacitive behavior in a porous electrode. This model was applied to the frequency region before the appearance of the “tail” in order to deconvolute the surface film resistance and the charge-transfer resistance during cycling. The extracted resistance values are shown in Figure 1e. Although the surface film resistance stabilized after 5 cycles, the baseline cell exhibited a continuous increase in the charge-transfer resistance over cycling. This increase in the electrode impedance was also observed by the large capacity drop at high current densities. The baseline cell lost over 50% of its initial capacity when discharging under a 5C current (Figure 1f). It is presented above that the parasitic reactions play a critical role in the electrochemical performance of battery cathodes and the rate of these side reactions significantly increases with the applied potential. In a conventional electrolyte, the electrochemical oxidation of solvent molecules can cause a thickening of the surface layer. The oxidation of solvent can also generate a high concentration of protons on the surface of the working electrode.21-22 We believe these generated protons can quickly react with the delithiated cathode materials, corroding and leaching the transition metals at a high potential.23 Both factors mentioned above significantly influence the cycling stability and impedance of the cathode. Based on these hypothesis, we could introduce two different types of protection routes for high-voltage cathodes: (i) a direct reduction in the catalytic decomposition of the electrolyte solvent; and (ii) formation of a


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“corrosion inhibitor film” that prevents severely attacking from protons even though the oxidation of solvent is not mitigated. With this in mind, we will validate our proposed mechanisms using two different known additives in the following sections. Mechanism (i): effect of LiDFOB additive In this section, we will use LiDFOB as a demonstration for our proposed mechanism (i). LiDFOB was first proposed as a conducting lithium salt by combining the merits of lithium bis(oxalato)borate (LiBOB) and LiBF4.24 The lower anodic stability (

Mechanistic Study of Electrolyte Additives to Stabilize High-Voltage

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