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Monitoring Changes in Electrolyte Composition of Commercial Li-Ion Cells after Cycling using NMR Spectroscopy and Differential Thermal Analysis
使用核磁共振波谱和差热分析监测循环后商用锂离子电池电解质组成的变化

, , and
FM Maddar 1 、R. Genieser 2 、CC Tan 1 和 MJ Loveridge 1

Published 20 March 2023 © 2023 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited
发布时间 20 三月 2023 • © 2023 作者.由IOP Publishing Limited代表The Electrochemical Society出版

, ,
电化学学会杂志,第 170 卷,第 3 期
Citation F. M. Maddar et al 2023 J. Electrochem. Soc. 170 030522
引自:F. M. Maddar et al 2023 J. Electrochem.Soc. 170 030522
DOI 10.1149/1945-7111/acc365
DOI: 10.1149/1945-7111/acc365

1945-7111/170/3/030522

Abstract 抽象

We illustrate a simple and effective electrolyte extraction methodology from commercial 18650 lithium-ion cells. This methodology is based on a liquid-liquid extraction step, which is highlighted for robustness, reproducibility, and reliability. We assess the consumption of electrolyte by tracking compositional changes using liquid-state nuclear magnetic resonance (NMR) spectroscopy, supported by differential thermal analysis (DTA) before and after cell cycling. An analysis method that monitors compositional dynamics is presented and shows the impact of these changes throughout a cell's lifetime. Such methodology can be employed in the understanding of electrolyte degradation mechanisms to enhance the understanding of performance fade in commercial cells. Moreover, it will help build robust mathematical models that are able to predict the drive of cell degradation and ultimate failure.
我们展示了一种从商用 18650 锂离子电池中提取简单有效的电解质的方法。该方法基于液-液萃取步骤,该步骤的稳定性、可重复性和可靠性突出。我们通过使用液态核磁共振 (NMR) 波谱跟踪成分变化来评估电解质的消耗,并得到电池循环前后的差热分析 (DTA) 的支持。提出了一种监测成分动力学的分析方法,并显示了这些变化在细胞生命周期中的影响。这种方法可用于理解电解质降解机制,以增强对商业电池性能衰减的理解。此外,它还将有助于建立强大的数学模型,能够预测细胞退化和最终失效的驱动因素。

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This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.
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Batteries are ubiquitous, and their domain of application is constantly growing. In particular, lithium-ion battery (LIB) technologies have surged in growth due to their combined unique features of high energy, power density, fast charging capabilities and long cycle life. 1,2 Initially, they were intensively pursued for a wide range of consumer electronics including cell phones, laptops, and power tools. More recently, their spectrum of application has diversified, and LIBs are continuing to make impacts in electric vehicle and grid energy storage markets. Uptake is also anticipated to grow in the sectors of aerospace and marine applications. 3,4
电池无处不在,其应用领域也在不断扩大。特别是,锂离子电池(LIB)技术由于其高能量、功率密度、快速充电能力和长循环寿命等独特特点而迅速增长。 1,2 最初,它们被广泛用于各种消费电子产品,包括手机、笔记本电脑和电动工具。最近,它们的应用范围已经多样化,锂离子电池继续在电动汽车和电网储能市场产生影响。预计航空航天和海洋应用领域的使用量也将增长。 3,4

However, despite their excellent properties, LIBs are susceptible to gradual capacity fade and limited lifetime, caused by a variety of complex degradation processes. 57 Some of these processes include Li deposition, 8 electrolyte decomposition, 9,10 particle cracking, 11 phase changes of electrode materials 12 and the formation of the passivating solid electrolyte interface (SEI) layer over the electrode surfaces. 13 Herein, we focus on examining the degradation behaviour of electrolytes in commercial, cylindrical LIBs. The electrolyte is in contact with all internal components of the cell and the properties of each of these affects the cell's overall operational performance. Moreover, the SEI is composed of electrolyte decomposition products, and so its properties are dependent on the composition of the electrolyte used. 14 Thus, understanding electrolyte decomposition and compositional dynamics in the overall degradation process is fundamental to future battery development. It also plays a crucial role in the safety and durability of LIBs by enabling predictions to be made around failure trajectories.
然而,尽管锂离子电池具有优异的性能,但由于各种复杂的降解过程,锂离子电池容易逐渐衰减和寿命有限。 57 其中一些过程包括锂沉积、 8 电解质分解、 9,10 颗粒开裂、 11 电极材料 12 的相变以及电极表面钝化固体电解质界面 (SEI) 层的形成。 13 在此,我们重点研究电解质在商业圆柱形锂离子电池中的降解行为。电解质与电池的所有内部组件接触,每个组件的特性都会影响电池的整体运行性能。此外,SEI由电解质分解产物组成,因此其性质取决于所用电解质的组成。 14 因此,了解整个降解过程中的电解质分解和成分动力学是未来电池开发的基础。它还通过能够围绕故障轨迹进行预测,在锂离子电池的安全性和耐久性方面发挥着至关重要的作用。

Generally, the electrolytes used in LIBs consist of a conducting salt - typically being lithium hexafluorophosphate (LiPF6) - dissolved in a mixture of linear and cyclic alkyl carbonate-based solvents including dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC) or propylene carbonate (PC). These solvents provide combined benefits, such as electrochemical stability over a wide potential window, broad operating temperature range, and the ability to form a stable protective SEI layer during cycling. 15 Additionally, specific additives are often added to the electrolyte to improve distinct properties; 16 many have reported the use of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) highlighting their effectiveness in improving the overall cell cycle life performance. 1721
通常,锂离子电池中使用的电解质由导电盐(通常是六氟磷酸锂(LiPF 6 ))组成,溶解在直链和环烷基碳酸酯基溶剂的混合物中,包括碳酸二甲酯(DMC)、碳酸二乙酯(DEC)、碳酸甲乙酯(EMC)、碳酸乙烯酯(EC)或碳酸丙烯酯(PC)。这些溶剂具有综合优势,例如在宽电位窗口内的电化学稳定性、宽工作温度范围以及在循环过程中形成稳定的保护性 SEI 层的能力。 15 此外,通常将特定的添加剂添加到电解质中以改善不同的性能; 16 许多人报道了碳酸亚乙烯酯 (VC) 和氟碳酸乙烯酯 (FEC) 的使用,突出了它们在改善整体细胞周期寿命性能方面的有效性。 1721

When LIBs are manufactured, under operation or in storage, the electrolyte undergoes compositional changes and partially decomposes electrochemically, chemically and/or thermally, forming various types of decomposition products. 22
当锂离子电池被制造、运行或储存时,电解质会发生成分变化,并在电化学、化学和/或热方面部分分解,形成各种类型的分解产物。 22

For example, the formation process during the first charge involves the decomposition of both solvents and salts to form a passivating SEI layer at the anode, which irreversibly changes the starting composition of the electrolyte. Therefore, reverse engineering and determination of the original mixtures used by cell manufacturers is difficult to achieve. Additional reactions like transesterifications or oligomerisations are susceptible to the presence of certain additives and other decomposition products and further complicate a detailed understanding of the system. 18,23 This makes the exact electrolyte degradation pathways challenging to elucidate.
例如,第一次充电期间的形成过程涉及溶剂和盐的分解,以在阳极形成钝化SEI层,这不可逆地改变了电解质的起始成分。因此,电池制造商使用的原始混合物的逆向工程和测定很难实现。酯交换或齐聚等其他反应容易受到某些添加剂和其他分解产物的影响,并使对系统的详细理解进一步复杂化。 18,23 这使得确切的电解质降解途径难以阐明。

To date, various studies have been performed to understand the decomposition of the electrolytes by analysing the composition of the gases generated or the direct study of the liquid electrolyte. These have used techniques such as gas chromatography (GC), 2427 ion chromatography (IC), 28 infrared spectroscopy (IR), 29,30 high performance liquid chromatography (HPLC), electrospray ionization mass spectrometry (ESI-MS) 31,32 and nuclear magnetic resonance (NMR) spectroscopy. 33,34 On the other hand, studying the composition and morphology of the resulting SEI on carbon surfaces can provide an indirect determination of the decomposition products. 35
迄今为止,已经进行了各种研究,通过分析产生的气体的组成或直接研究液体电解质来了解电解质的分解。这些技术包括气相色谱法(GC)、 2427 离子色谱法(IC)、 28 红外光谱法(IR)、 29,30 高效液相色谱法(HPLC)、电喷雾电离质谱法(ESI-MS) 31,32 和核磁共振(NMR)波谱法。 33,34 另一方面,研究所得SEI在碳表面上的组成和形貌可以间接测定分解产物。 35

Extracting electrolytes directly from commercial cylindrical cells is challenging, as it has permeated into the electrodes and separator pores, thus not be readily available for further analysis. 36 Some circumvent this by immersing the materials in a suitable solvent for a certain amount of time, followed by filtration of the extracted electrolyte. 37 Other approaches exist in which the whole jelly roll is transferred into an external container and is centrifuged for some time. 27 However, these methods are susceptible to solvent evaporation resulting in compositional changes that are difficult to quantify. A number of studies have reported on the extraction of electrolytes from the jelly roll of cylindrical Li-ion cells with supercritical and subcritical CO2 38,39 while others report on the solid phase microextraction complemented with GC-MS. 40 Herein, we build on the extraction methodology used by Petibon et al. in which they have studied small pouch cells (220–320 mAh). 37 We focus on the extraction of electrolyte from commercially available 18650 cylindrical Li-ion cells (18650). Using NMR, we were uniquely able to monitor electrolyte compositional changes at different stages of cell cycling and obtain information about component consumption. NMR has proven to be a very powerful tool to investigate LIB electrolyte stability in a qualitative and quantitative manner of both its organic solvents and inorganic salt constituents. 34 Additionally, differential thermal analysis (DTA) was used as a complimentary technique, as shown previously by Dhan et al. as a powerful non-destructive method to study electrolyte behaviour. 41 Tracking the electrolyte at different points of the cell's cycle life can help elucidate its degradation process and the overall understanding of cell failure.
直接从商业圆柱形电池中提取电解质具有挑战性,因为它已经渗透到电极和隔膜孔中,因此不容易用于进一步分析。 36 有些人通过将材料浸入合适的溶剂中一定时间,然后过滤提取的电解质来规避这一点。 37 存在其他方法,其中将整个果冻卷转移到外部容器中并离心一段时间。 27 然而,这些方法容易受到溶剂蒸发的影响,导致难以量化的成分变化。许多研究报道了用超临界和亚临界一氧化碳 2 从圆柱形锂离子电池的果冻卷中提取电解质, 38,39 而其他研究则报道了辅以GC-MS的固相微萃取。 40 在此,我们建立在Petibon等人使用的提取方法之上,他们研究了小袋细胞(220-320 mAh)。 37 我们专注于从市售的 18650 圆柱形锂离子电池 (18650) 中提取电解质。使用核磁共振,我们能够独特地监测电池循环不同阶段的电解质组成变化,并获得有关组分消耗的信息。核磁共振已被证明是一种非常强大的工具,可以定性和定量地研究LIB电解质有机溶剂和无机盐成分的稳定性。 34 此外,差分热分析(DTA)被用作一种补充技术,正如Dhan等人之前所表明的那样,这是一种研究电解质行为的强大无损方法。 41 在电池循环寿命的不同点跟踪电解质有助于阐明其降解过程和对电池故障的整体理解。

Experimental 实验的

Materials 材料

All cells tested herein were commercial LG 18650 high power cells in which their capacity was checked prior to the electrolyte extraction. Deuterated chloroform (CDCl3) (>= 99% purity) and Tetramethylsilane (TMS) (ACS reagent) NMR grade, (>=99.9% purity) were purchased from Sigma-Aldrich. Wilmad (R) low pressure NMR tubes (300 MHz frequency) were used that were also purchased from Sigma-Aldrich.
本文测试的所有电池均为商用 LG 18650 高功率电池,在提取电解液之前检查其容量。氘代氯仿(CDCl 3 )(>=99%纯度)和四甲基硅烷(TMS)(ACS试剂)NMR级(>=99.9%纯度)购自Sigma-Aldrich。使用了Wilmad(R)低压NMR管(300 MHz频率),这些管也是从Sigma-Aldrich购买的。

Cycling protocol 循环协议

Cells were cycled using Maccor S4000 in a BINDER climate chamber set to 40 °C. Cells were charged and discharged at 40 °C between 2.5 − 4.2 V with a multistage constant current (MCC) fast charging protocol. This consisted of 1.45 C charging to 40% state of charge (SOC), 1.2 C to 60% SOC and 0.7 C to 80% SOC based on coulomb counting, followed by a C/3 charge to 4.2 V. This was then followed by a constant voltage (CV) step with current tapered to C/10. The discharge was carried out at C/3 to 0% SOC. After every 23 cycles of fast charging, 2 benchmark cycles were performed at constant current of C/5 for charge and discharge with a CV step until the current is tapered to C/10. The benchmark cycles were used to adjust the capacity rating of the cell in order to maintain the fast charge SOC steps via coulomb counting.
使用Maccor S4000在设置为40°C的BINDER气候室中循环细胞。 电池在40°C的2.5 − 4.2 V之间使用多级恒流(MCC)快速充电协议进行充电和放电。这包括 1.45 C 充电至 40% 充电状态 (SOC)、1.2 C 充电至 60% SOC 和 0.7 C 充电至 80% SOC(基于库仑计数),然后 C/3 充电至 4.2 V。然后是恒压 (CV) 步进,电流逐渐减少到 C/10。放电在 C/3 至 0% SOC 下进行。在每 23 个周期的快速充电后,以 C/5 的恒定电流执行 2 个基准循环,以 CV 步长进行充电和放电,直到电流逐渐减小到 C/10。基准循环用于调整电池的额定容量,以便通过库仑计数保持快速充电 SOC 步长。

Solvent extraction and sample preparation
溶剂萃取和样品制备

Electrolyte was extracted from fresh (as received - uncycled) and cycled cells that have gone through different cycling durations: 100, 200, 300, 600, 900, 1200 and 1600 cycles. For each cycle number, two cells were opened. All samples were prepared in an argon atmosphere glovebox (<5 ppm H2O/O2). The top cap of the battery cell was opened using a pipe cutting tool, allowing the exposure of the top surface of the cell jelly roll shown in Fig. 1A. The opened cell was then immediately placed in an HDPE (high density polyethylene) vial containing 10 g CDCl3 with the exposed top part facing down to extract as much of the electrolyte and avoid any evaporation. CDCl3 was used as the extraction solvent to acquire only the soluble organic components of the electrolyte and leave out its insoluble salt constituents. Note, monitoring both the solvent and salt consumption over cycle life would be of great value. Therefore, DMSO was also tested but showed to react with components within the cell resulting in electrolyte discolouration, thus CDCl3 was the only solvent used through this study. The advantage of extracting only the organic components is that it allows the electrolyte to be further analysed if required with other complementary systems such as GC-MS that is susceptible to damage by corrosive fluoride-based species. The cell was allowed to mix continuously for 24 h in a tubular mixer making sure the cell is positioned vertically along its axis as shown in Fig. 1B, to maximise interaction between the solvent and the jelly roll. It is crucial to maintain a suitable mixing time as it can influence the extracted amount. As the basis of our methodology, we have chosen the duration of mixing to be 24 h as no significant change was observed when mixed for 48 h, shown in the Supporting Information (SI), Fig. 1S. This works towards an efficient and effective mixing time whereby the solvent is able to reach all the pores resulting in a homogeneous mixture. Following the mixing procedure, the solution was filtered using Whatman Spartan syringe filters (SPARTAN 30/0.45 RC filter unit). 1 g of reference standard (consisting of TMS: CDCl3 in 1:5 ratio) was added to 5 g of the filtrate, as shown in Fig. 1C. As both TMS and CDCl3 are volatile solvents, the reference standard was freshly prepared each time. The final extracted solution containing the reference standard was then directly transferred into an NMR tube (Fig. 1D). All 1H-NMR analysis were conducted on a Bruker Avance III HD 300 MHz spectrometer. The NMR data processing was done using TopSpinTM (Bruker, USA). Each spectrum was shifted against TMS peak at 0 ppm.
电解质是从新鲜的(接收的 - 未循环的)和循环电池中提取的,这些电池经历了不同的循环持续时间:100、200、300、600、900、1200 和 1600 次循环。对于每个周期数,打开两个单元格。所有样品均在氩气气氛手套箱(<5 ppm H 2 O / O 2 )中制备。使用管道切割工具打开电池的顶盖,允许暴露图1A所示的电池果冻卷的顶面。然后立即将打开的电池放入含有10g CDCl 3 的HDPE(高密度聚乙烯)小瓶中,暴露的顶部朝下,以提取尽可能多的电解质并避免任何蒸发。使用CDCl 3 作为萃取溶剂,仅获取电解质的可溶性有机组分,而忽略其不溶性盐成分。请注意,在循环寿命期间监测溶剂和盐的消耗量将具有重要价值。因此,DMSO也进行了测试,但显示与电池内的成分发生反应,导致电解质变色,因此CDCl 3 是本研究使用的唯一溶剂。仅提取有机成分的优点是,如果需要,可以与其他互补系统(如易受腐蚀性氟化物基物质损害的GC-MS)一起进一步分析电解质。允许将细胞在管式混合器中连续混合24小时,确保细胞沿其轴垂直定位,如图1B所示,以最大限度地提高溶剂和果冻卷之间的相互作用。保持合适的混合时间至关重要,因为它会影响提取量。 作为我们方法的基础,我们选择混合的持续时间为24小时,因为混合48小时时没有观察到显着变化,如支持信息(SI)所示,图1S。这有助于实现高效和有效的混合时间,从而使溶剂能够到达所有孔隙,从而形成均匀的混合物。按照混合程序,使用Whatman Spartan注射器过滤器(SPARTAN 30/0.45 RC过滤装置)过滤溶液。将1 g参比标准品(由TMS:CDCl3组成,比例为1:5)加入到5 g滤液中,如图1C所示。由于TMS和CDCl 3 都是挥发性溶剂,因此每次都要新鲜制备参考标准品。然后将含有参考标准品的最终萃取溶液直接转移到NMR管中(图1D)。所有 1 H-NMR 分析均在 Bruker Avance III HD 300 MHz 波谱仪上进行。使用TopSpinTM(美国布鲁克)进行NMR数据处理。每个光谱在0 ppm时与TMS峰相比偏移。

Figure 1.

Figure 1. Schematic illustrating the method used to extract solvents from 18650 cells.
图 1.示意图说明了用于从 18650 个电池中提取溶剂的方法。

Standard image High-resolution image

Differential thermal analysis (DTA)
差热分析(DTA)

DTA measurements were performed on 8 cylindrical cells (1 fresh and 7 cycled cells of which gone through: 100, 200, 300, 600, 900, 1200 and 1600 cycles), with a dry Novonix cell used as a reference. OCV of all cells was close to 3.53 V +/− 0.01 V). All the cells were subjected to the following three testing conditions: (i) from room temperature, cool to −150 °C at a rate of 3 °C min−1; (ii) hold isothermally at −150 oC for 20 min and (iii) heat to 45 oC at a rate of 0.5 oC min−1. Testing was carried out onsite at Novonix Testing Services, Bedford, Canada. 42 The DTA signal originates from the difference in measured temperature during phase transition between the sample test cell at each cycle number and a reference uncycled fresh cell of the same type (Tsample – Treference). 41
对 8 个圆柱形电池(1 个新鲜电池和 7 个循环电池,其中经历了:100、200、300、600、900、1200 和 1600 次循环)进行了 DTA 测量,以干燥的 Novonix 电池为参考。所有电池的OCV接近3.53 V +/− 0.01 V)。所有细胞都经受以下三种测试条件:(i)从室温,以3°Cmin的速率冷却至-150°C −1 ;(ii)在-150 o °C下保持等温20分钟,(iii)以0.5 o °C的速率加热至45 o ° −1 C。测试在加拿大贝德福德的 Novonix 测试服务公司现场进行。 42 DTA信号来源于每个循环次数的样品测试池与相同类型(T-T sample reference )的参考未循环新鲜池之间相变期间的测量温度差。 41

Results and Discussion 结果与讨论

Reproducibility of the extraction procedure
提取过程的可重复性

To investigate the robustness of the extraction method, electrolyte was extracted from five fresh cells (A-E) of the same type (LG 18650). Both GCMS (in SI, Fig. 2S) and NMR were used to obtain the composition of the electrolyte. Figure 2 shows the acquired NMR spectra. The major solvent peaks were found to be DMC, EC, PC, EMC, DEC with succinonitrile (SN) and tris(trimethylsilyl)phosphate (TMPS) as additives. SN acts as a corrosion inhibitor of the copper current collector as well as improving thermal stability in EC-based electrolytes. 43,44 SN has also been shown to improve cell performance significantly at high voltages by forming a more stable and efficient SEI film. 45 TMPS has been used in Li-rich cathodes to stabilise the electrode/electrolyte interface resulting in improved cycling performance. 46 TMPS oxidises more readily as compared to other electrolyte components. In addition to this, it is highly reactive towards HF and acts as an HF scavenger, thereby improving the electrochemical performance of LIBs. 47 A singlet peak at ∼3.53 ppm was also observed consisting of lithium ethylene dicarbonate (LEDC) resulting from EC reduction illustrated in Fig. 2D, as previously reported. 48 NMR signals derived from EMC and DEC are difficult to separate as they result in overlapping chemical shifts. Additionally, as shown in Fig. 2B, the triplet peak overlaps with a singlet peak derived from CDCl3 impurities making it difficult to quantify. Moreover, the quartet peaks of EMC and DEC shown in Fig. 2G were very small in nature (∼0.2% weight relative to other solvents) and require a higher resolution NMR to obtain sharper signals for quantification. Thus, the relative peaks of DMC (singlet at ∼3.78 ppm), PC (doublet at ∼1.5 ppm), EC (singlet at ∼4.5 ppm), SN (singlet at ∼2.78 ppm), TMPS (singlet at ∼0.19 ppm) and LEDC (singlet at ∼3.5 ppm) were the ones that were quantified and compared throughout. Table I illustrates the relative percentage weights present in the overall electrolyte extracted from 5 fresh cells (see SI3 for detailed information of the associated peaks). Similar compositions were observed for each cell with the electrolyte being dominated by DMC highlighting the reliability of the extraction method used.
为了研究提取方法的稳健性,从五个相同类型的新鲜细胞 (AE) (LG 18650) 中提取电解质。GCMS(SI中,图2S)和NMR均用于获得电解质的组成。图2显示了采集的NMR波谱。主要溶剂峰为DMC、EC、PC、EMC、DEC,以丁二腈(SN)和磷酸三(三甲基硅基)酯(TMPS)为助剂。SN可作为铜集流体的缓蚀剂,并改善EC基电解质的热稳定性。 43,44 SN还被证明可以通过形成更稳定和高效的SEI薄膜来显着提高电池在高压下的性能。 45 TMPS已用于富锂阴极,以稳定电极/电解质界面,从而改善循环性能。 46 与其他电解质成分相比,TMPS更容易氧化。除此之外,它还对 HF 具有高度反应性,并充当 HF 清除剂,从而提高 LIB 的电化学性能。 47 如前所述,还观察到 ∼3.53 ppm 的单线态峰,由图 2D 所示的 EC 还原产生的碳酸乙烯酸锂 (LEDC) 组成。 48 来自EMC和DEC的NMR信号很难分离,因为它们会导致重叠的化学位移。此外,如图2B所示,三重态峰与来自CDCl 3 杂质的单重态峰重叠,因此难以定量。此外,图2G所示的EMC和DEC四重峰在性质上非常小(相对于其他溶剂的重量约为0.2%),需要更高分辨率的NMR才能获得更清晰的定量信号。因此,DMC(单重态,∼3.78 ppm)、PC(双重态,∼1。5 ppm)、EC(单线态 ∼4.5 ppm)、SN(单线态 ∼2.78 ppm)、TMPS(单线态 ∼0.19 ppm)和 LEDC(单线态 ∼3.5 ppm)是量化和比较的。表I说明了从5个新鲜电池中提取的总电解质中存在的相对百分比权重(有关相关峰的详细信息,请参见SI3)。观察到每个电池的类似成分,电解质以DMC为主,突出了所用提取方法的可靠性。

Figure 2.

Figure 2. NMR spectra acquired from five fresh cells (A-E) of the same type (LG 18650).
图2.从五个相同类型的新鲜细胞 (A-E) (LG 18650) 获得的 NMR 波谱。

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Table I. The relative percentage composition found in electrolyte extracted from five fresh cells (A-E) of the same type (LG 18650).
表一.从五个相同类型的新鲜细胞 (AE) 中提取的电解质中发现的相对百分比组成 (LG 18650)。

Relative % weight 相对重量百分比ABCDEAvg. 平均Std. Dev. 标准开发
PC4.023.923.943.863.973.94±0.06
DMC73.9173.2472.9074.8571.6673.31±1.18
EC18.9619.6319.3718.3520.9019.44±0.95
LEDC1.160.981.080.980.971.03±0.08
SN0.440.390.470.420.490.44±0.04
TMPS1.501.842.251.552.001.83±0.31

Monitoring compositional changes of electrolyte during cycling
监测循环过程中电解质的组成变化

Ordinarily, Li-ion batteries exhibit a slow capacity degradation, until they reach an inflection point - whereby the capacity fade becomes much more rapid and drops off quickly as shown in the capacity retention and the associated cycle efficiency plots illustrated in Fig. 3i. This is commonly referred to as a knee-point. 49 It has been shown that a knee-point marks the rapid capacity fade trend to the end-of-life (EOL). 50 Herein, electrolyte was extracted from various cells at different stages of the cell's cycle life before and after the knee-point. Figure 3i illustrates the multiple points at which electrolyte was extracted, at cycle numbers 100, 200, 300, 600 and 900 being above the 80% capacity retention mark and 1200 and 1600 falling below that mark. Cells were then disassembled and as illustrated in Fig. 3ii, the presence of dense lithium plating on the anode electrode surface was observed. Accordingly, this could indicate its contribution to the poor cycling efficiency seen from ∼1000 cycles as it continues to form throughout cycling.
通常,锂离子电池表现出缓慢的容量下降,直到它们达到拐点——此时容量衰减变得更快并迅速下降,如图 3i 所示的容量保持和相关循环效率图所示。这通常被称为膝盖点。 49 研究表明,拐点标志着产能快速衰减趋势,直至寿命终结 (EOL)。 50 本文中,电解质是从处于细胞循环寿命的不同阶段的各个细胞中提取的,在膝盖点之前和之后。图3i显示了提取电解液的多个点,循环编号100、200、300、600和900高于80%容量保持标记,1200和1600低于该标记。然后拆卸电池,如图3ii所示,观察到阳极电极表面存在致密的锂镀层。因此,这可能表明它导致了 ∼1000 次循环中观察到的不良循环效率,因为它在整个循环过程中继续形成。

Figure 3.

Figure 3. (i) Capacity retention and corresponding cycle efficiency plot as a function of cycle number, highlighting the different stages (100, 200, 300, 600, 900, 1200 and 1600 cycles) at which the electrolyte is extracted from the cell and (ii) post-mortem observation of the anode surface after 1600 cycles.
图3.(i) 容量保持率和相应的循环效率图作为循环次数的函数,突出显示了从电池中提取电解质的不同阶段(100、200、300、600、900、1200 和 1600 次循环),以及 (ii) 1600 次循环后阳极表面的尸检观察。

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To observe and quantify the consumption of individual components, the peaks for DMC, EC, PC, LEDC, as well as SN and TMPS additives were analysed against the standard TMS peak, as shown in Fig. 4i. Hence, an initial increase in the peak areas was seen for DMC, PC, EC and LEDC relative to the TMS as illustrated in Fig. 4ii. This is likely due to the use of cells of a different batch number relative to the fresh cells, highlighting the sensitivity of the measurements. This was then followed by a gradual decrease over cycle number. TMPS showed a steady decrease over cycle number and stabilises after 800 cycles, whereas SN indicated a much more evident decrease at an exponential rate throughout the cycle life and was completely used up at cycle number 1600. The observed evidence of the constant decrease of SN through the cycles, can therefore be deduced that SN has a significant role in improving the cell's overall cycle life. Total disappearance of SN coincides with the occurrence of a knee-point – this further supports the significance of its role. Interestingly, the peak for LEDC stayed consistent throughout cycle number. LEDC was shown to be one of the main compositions of the SEI layer formed on graphite anodes in EC and EMC based electrolytes and have experienced no change in its composition upon cycling. 48 This illustrates the effectiveness of the SEI layer containing LEDC by preventing further electrolyte reduction. In addition, we observed the increase of NMR peaks in the region around ∼5.77 and 5.64 ppm as illustrated in Fig. 5. This is a representation of degradation products as the cells were cycled originating from the decomposition products of DMC attributed to acetal species, such as methanediol and methoxymethanol. 51
为了观察和量化单个组分的消耗量,根据标准TMS峰分析了DMC、EC、PC、LEDC以及SN和TMPS添加剂的峰值,如图4i所示。因此,DMC、PC、EC 和 LEDC 相对于 TMS 的峰面积最初增加,如图 4ii 所示。这可能是由于使用了与新鲜细胞不同批号的细胞,突出了测量的灵敏度。然后,循环次数逐渐减少。TMPS在循环次数上稳步下降,并在800次循环后稳定下来,而SN在整个循环寿命中以指数速率下降更为明显,并在循环次数1600时完全用完。因此,观察到SN在循环过程中不断减少的证据可以推断出SN在改善细胞的整体循环寿命方面具有重要作用。SN的完全消失与膝点的发生相吻合,这进一步支持了其作用的重要性。有趣的是,LEDC的峰值在整个周期数中保持一致。LEDC被证明是EC和EMC基电解质中在石墨阳极上形成的SEI层的主要成分之一,并且在循环时其成分没有变化。 48 这说明了含有LEDC的SEI层通过防止电解质进一步还原的有效性。此外,我们观察到该区域的NMR峰增加,约为∼5.77和5.64 ppm,如图5所示。这是降解产物的表示,因为细胞的循环起源于归因于缩醛物种的DMC分解产物,例如甲烷二醇和甲氧基甲醇。 51

Figure 4.

Figure 4. (i) NMR spectra of electrolyte extracted from a fresh cell and cycled cells (100–1600). (ii) Relative integrals of electrolyte components against TMS over cycle life showing the overall solvent trend. Note, cycle number 0 represents a cell that has just gone through formation and has not been cycled.
图4.(i) 从新鲜细胞和循环细胞中提取的电解质的核磁共振波谱 (100–1600)。(ii) 电解质组分在循环寿命内与 TMS 的相对积分,显示了总体溶剂趋势。请注意,循环编号 0 表示刚刚形成且尚未循环的细胞。

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