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2014-2018年Jeff Dahn课题组应用ARC相关研究小结

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

加拿大达尔豪斯大学的Jeff Dahn教授是三元材料LiNixMnyCozO2 (x+y+z=1, NMC)工业化应用的先驱,在锂电领域硕果累累。Dahn教授自开始学术研究生涯就同学术界保持紧密联系:1990-1996年,任职加拿大西蒙弗雷泽大学(Simon Fraser University),同时是E-One Moli Energy公司一员;1996-2006年,任职加拿大达尔豪斯大学(Dalhousie University),同时是3M公司先进材料研究的专家顾问,时间长达20年;2006年至今,任职加拿大达尔豪斯大学(Dalhousie University),同特斯拉签署5年合作协议,提升电池能量密度和寿命,同时降低电池成本。也正因为和工业界联系紧密,Dahn课题组研究很接地气,因此在锂电领域有着极高的声誉。

注:NMC原始专利就是Dahn同3M合作期间于2001年申请的。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

ARC (Accelerating Rate Calorimetry)已被广泛用于锂离子电池及相关材料的热性能研究。本篇拟对Dahn课题组近几年利用ARC研究电解液添加剂改善效果和NMC热稳定性的相关工作进行小结。

1. Lin Ma, Jian Xia, Xin Xia, J. R. Dahn. The Impact of Vinylene Carbonate, Fluoroethylene Carbonate and Vinyl Ethylene Carbonate Electrolyte Additives on Electrode/Electrolyte Reactivity Studied Using Accelerating Rate Calorimetry. Journal of The Electrochemical Society, 161 (10) A1495-A1498 (2014).

链接:

http://jes.ecsdl.org/content/161/10/A1495.abstract

要点:(1)VC、FEC和VEC对脱锂态NCM333和电解液之间的反应活性没有影响;(2)VEC对嵌锂态石墨和电解液之间的反应活性没有影响;(3)VC的加入能提高嵌锂态石墨和电解液之间的稳定性;(4)FEC能降低嵌锂态石墨和电解液之间的自产热速率;(4)2%添加量的VC或FEC对嵌锂态石墨和电解液之间反应活性影响不大。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 1. Molecular structures of the electrolyte additives used (a) vinylene carbonate (VC) (b) fluoroethylene carbonate (FEC) and (c) vinyl ethylene carbonate (VEC).

本工作主要利用ARC研究使用VC、FEC和VEC三种电解液添加剂对电极和电解液之间反应活性的影响。VC、FEC和VEC是锂离子电池中常用的电解液添加剂,其分子结构式如图1所示。由于ARC实验的正负极粉体由扣电获得,经过DMC清洗和干燥过程。其中正极为NMC333,充电至4.2 V;负极为MCMB,放电至0.005 V。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 3. Self-heating rate vs. temperature for delithiated NMC reacting with 1M LiPF6 in EC/DEC with 10 wt% VC (a), with 10 wt% VEC (b), and with 10 wt% FEC (c) compared with the control electrolyte.

Figure 4. Self-heating rate vs. temperature for lithiated graphite reacting with 1M LiPF6 in EC/DEC with 10 wt% VC (a), with 10 wt% VEC (b), with 10 wt% FEC (c) compared with the control electrolyte. Dashed frames emphasize the temperature range (80–200 ℃). These experiments used 70 mg lithiated graphite and 70 mg electrolyte.

图3为电解液中VC、VEC和FEC添加量分别为10%时脱锂态NMC333同电解液在不同温度下的SHR(Self-heating rate,自加热率)曲线。总体看VC、VEC和FEC的添加对脱锂态NCM333和电解液之间反应的SHR曲线没有显著影响,在275 ℃之前SHR均达到峰值。

图4为电解液中VC、VEC和FEC添加量分别为10%时嵌锂态石墨同电解液在不同温度下的SHR曲线。10%VC的加入显著降低了在100 ℃附近的放热,在80-150 ℃范围均降低了自放热率,但在180-250 ℃均提高了自放热率(图4a)。10%VEC的加入对自放热率没有显著影响(图4b)。10%FEC的加入不仅降低了自放热的起始温度,同时在160-250 ℃范围显著提高了自放热率。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 5. Self-heating rate vs. temperature for lithiated graphite reacting with 1M LiPF6 in EC/DEC with 2 wt%, 5 wt% and 10 wt% VC compared with the control electrolyte. These experiments used 140 mg lithiated graphite and 140 mg electrolyte.

Figure 6. Self-heating rate vs. temperature for lithiated graphite reacting with 1M LiPF6 in EC/DEC with 2 wt%, 5 wt% and 10 wt% FEC compared with the control electrolyte. These experiments used 140 mg lithiated graphite and 140 mg electrolyte.

图5为不同VC添加量条件下嵌锂态石墨和电解液在不同温度下的SHR曲线。2%VC添加量时,100 ℃左右的放热峰依然存在,但较控制组有显著较低;5%和10%VC添加量时,该处放热峰几乎消失。但随着VC量的提高,130 ℃左右的放热峰反而提高。因此,高VC添加量有利于提高130 ℃以下嵌锂态石墨和电解液的稳定性,低VC添加量有利于提高130 ℃以上嵌锂态石墨和电解液的稳定性。

图6为不同FEC添加量条件下嵌锂态石墨和电解液在不同温度下的SHR曲线。可以看出FEC的加入使放热起始温度降低至50 ℃左右且降低了100 ℃左右的自产热速率。随着FEC含量的增加,嵌锂态石墨和电解液在130 ℃以上的自产热速率不断增大。

2. Lin Ma, Jian Xia, J. R. Dahn. Ternary Electrolyte Additive Mixtures for Li-Ion Cells that Promote Long Lifetime and Less Reactivity with Charged Electrodes at Elevated Temperatures. Journal of The Electrochemical Society, 162 (7) A1170-A1174 (2015).

链接:

http://jes.ecsdl.org/content/162/7/A1170.full

要点:三组分添加剂如VC-211、PES-211能提高NMC111、NMC442软包电池循环性能和电极/电解液化学稳定性,且效果远高于单一添加剂或二元添加剂。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 1. Chemical structures of the electrolyte additives used (a) prop-1-ene-1,3-sultone (PES) (b) methylene methanedisulfonate (MMDS) (c) ethylene sulfate (DTD) (d) 1,3-propylene sulfate (TMS) (e) propylene sulfate (PLS) (f) tris(trimethylsilyl) phosphate (TTSP), (g) tris(trimethylsilyl) phosphite (TTSPi) and (h) vinylene carbonate (VC).

Figure 2. (a) Capacity versus cycle number for NMC111/graphite pouch cells (unclamped) containing selected additives or additive blends. The cycling was done between 2.8 and 4.2 V at 55 ℃ and at 80 mA. (b) Capacity versus cycle number for NMC442/graphite pouch cells (unclamped) containing selected additives or additive blends. The cycling was done between 3.0 and 4.4 V at 45 ℃ and at 100 mA.

本工作主要考察不同添加剂组合对NMC111、NMC442软包电池性能和电极电解液之间反应活性的影响。考察的添加剂共8种,分别为PES、MMDS、DTD、TMS、PLS、TTSP、TTSPi和VC,其结构式如图1所示。图2a为不同添加剂组合下NMC111软包电池在55 ℃、2.8-4.2 V条件下的循环结果。可以看出效果最好的为三组分添加剂“PES-211”,即2%PES+1%MMDS+1%TTSPi,循环900圈容量保持率仍高于80%。图2b为不同添加剂组合下NMC442软包电池在45 ℃、3.0-4.4 V条件下的循环结果。三组分添加剂PES-211 (2%PES+1%MMDS+1%TTSPi)同样效果最好,性能远超2%VC或2%PES。

 

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 3. Self-heating rate vs. temperature for delithiated NMC reacting with 1.0 M LiPF6 in EC/EMC (3/7 wt%) with 2 wt% (a) VC, (b) PES, (c) MMDS, (d) DTD, (e) TMS, (f) PLS, (g) TTSP and (h) TTSPi compared with the control electrolyte.

图3为4.2 V脱锂态NMC和含2%不同添加剂电解液在不同温度下的自放热曲线。2%VC添加量下脱锂态NMC和电解液的自放热率率高于控制组;2%DTD、2%TMS、2%PLS或2%TTSP结果同对照组几乎一致;2%MMDS, 2%PES或2%TTSPi均能降低250 ℃以上脱锂态NMC和电解液反应的自放热率,热失控温度延后10 ℃左右。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 4. Self-heating rate vs. temperature for lithiated graphite reacting with 1.0 M LiPF6 in EC/EMC (3/7 wt%) with 2 wt% (a) VC, (b) PES, (c) MMDS, (d) DTD, (e) TMS, (f) PLS, (g) TTSP and (h) TTSPi compared with the control electrolyte.

    图4为嵌锂态石墨和含2%不同添加剂电解液在不同温度下的自放热曲线。粗略看,添加剂对负极的影响远大于对正极的影响。除了PES外,其他七种添加剂均能在全温度范围内降低自放热率。其中,2% MMDS存在时100 ℃左右几乎观察不到放热峰,表明石墨表面形成了高热稳定性膜;2%TTPSi存在时,150-200 ℃范围的自放热率相比控制组急剧降低。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 5. Self-heating rate vs. temperature for lithiated graphite reacting with 1.0 M LiPF6 in EC/EMC (3/7 wt%) with (a) “VC-211” and (b) “PES-211” compared with the control electrolyte.

图5为嵌锂态石墨和含VC-211(2%VC+1%MMDS+1%TTSPi)、PES-211(2%PES+1%MMDS+1%TTSPi)三组分添加剂电解液在不同温度下的自放热曲线。VC-211或PES-211的加入不仅能显著降低100 ℃左右的放热,同时能在几乎全温度范围内降低自放热率。

3. Lin Ma, Mengyun Nie, Jian Xia, J.R. Dahn. A systematic study on the reactivity of different grades of charged Li[NixMnyCoz]O2 with electrolyte at elevated temperatures using accelerating rate calorimetry. Journal of Power Sources, 2016, 327: 145-150.

链接:

https://www.sciencedirect.com/science/article/abs/pii/S0378775316309053

要点:(1)随着Ni含量或脱锂量(充电电压)增加,NMC材料反应活性不断提高;(2)NMC811反应活性远高于其他NMC材料,其安全性值得关注;(3)4.5 V NMC422克容量较高,同时反应活性相对较低,值得关注。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Table 1. Summary of the specific surface area for the positive electrode materials in the NMC111/graphite, NMC442/graphite, NMC532/graphite, NMC622/graphite and NMC811/graphite pouch cells. The instrumental error in each value is estimated to be ±0.03 m2/g.

Table 2. The chemical composition of the delithiated NMC samples, the molar mass, the full cell capacity and the positive electrode specific capacity for the different NMC grades charged to the different upper cut-off potentials.

本工作利用ARC系统研究了不同脱锂态NMC111、NMC442、NMC532、NMC622和NMC811同电解液(EC:EMC=3:7, 1.0 M LiPF6)的反应活性。六种NMC材料比表面积和不同脱锂态(充电电压)下材料信息分别如表1和表2所示。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Fig. 2. SHR vs. temperature for delithiated (a) NMC111, (b) NMC442, (c) NMC532, (d) NMC622 and (e) NMC811 reacting with control electrolyte at different cut-off voltages. The results for duplicate samples are given as dashed lines in each panel.

如图2a所示,4.2 V NMC111只有当温度达到200 ℃左右时才有显著的放热,当温度达到225 ℃左右自放热率急剧上升,而4.7 V NMC111只有当温度达到约180 ℃自放热率才急剧上升。不同电压NMC442同样能观察到类似NMC111的自放热现象,4.7 V NMC当温度达到约175 ℃时自放热率急剧上升。4.4 V和4.5 V NMC532的自放热率只是略高于4.2 V NMC532;当温度达到约160 ℃时,4.7 V NMC532自放热率急剧上升。4.2 V、4.4 V和4.5 V NMC622的起始自放热温度约为160 ℃,4.7 V NMC622的起始自放热温度约为150 ℃。最值得注意的是NMC811,4.2-4.7 V不同脱锂态NMC811自放热曲线几乎一致且120 ℃左右自放热率即急剧上升。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Fig. 3. SHR vs. temperature results for the different delithiated NMC grades reacting with control electrolyte at (a) 4.2 V, (b) 4.4 V, (c) 4.5 V and (d) 4.7 V. The results for duplicate samples are given as dashed lines in each panel.

对比图3不难发现:(1)4.2-4.5 V,NMC111和NMC442的自产热曲线几乎一致;(2)在相同充电电压下,NMC532和NMC622的反应活性高于NMC111和NMC422的活性,而NMC811的反应活性则远高于其他几种NMC材料

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Fig. 4. The temperature where the self-heating rate reaches 0.2 ℃/min plotted versus the remaining lithium content, x, in Lix[NMC]O2. This compares all the materials at the same degree of delithiation.

图4是对以上结果的汇总对比,其中纵坐标为自放热率达到0.2 ℃/min。4.5 V NMC442(x=0.25)反应活性低于4.4 V NMC532和4.2 V NMC811,NMC811和其他NMC材料完全不是一家兄弟,简直不要太猛。

4. Que Huang, Lin Ma, Aaron Liu, Xiaowei Ma, Jing Li, Jian Wang, J.R. Dahn. The reactivity of charged positive Li1-n[NixMnyCoz]O2 electrodes with electrolyte at elevated temperatures using accelerating rate calorimetry. Journal of Power Sources, 2018, 390: 78–86.

链接:

https://www.sciencedirect.com/science/article/abs/pii/S0378775318303793

要点:添加剂对正极材料和电解液反应活性几乎没有影响。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Fig. 1. Chemical structures of the electrolyte additives used in this work.

本工作旨在利用ARC研究不同脱锂态NMC材料同含不同添加剂电解液的化学反应活性。考察的四种添加剂分别为VC、DTD、MMDS和FEC,其分子结构式如图1所示。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Table 2. The chemical composition of the delithiated NMC grades, the molar mass, the full cell capacity and the specific capacity of the different positive electrodes charged to the different upper cut-off potentials.

考察的三种NMC材料分别为SC-NMC532(单晶532)、NMC622A(Al2O3包覆的NMC622)和NMC622B(包覆物未具体说明),该三种材料在不同脱锂态(充电电压)下的相关信息如表2所示。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Fig. 4. SHR vs. temperature for delithiated (a) SC-NMC532, (b) NMC622A and (c) NMC622B reacting with control electrolyte 1.09 mol/kg LiPF6 in EC: EMC (3:7) at different cut-off voltages. The results for duplicate samples are given as a dashed line in each panel.

图4为不同脱锂态三种NMC材料同不含添加剂电解液在不同温度下的自放热率曲线。4.1 V、4.2 V和4.3 V SC-NMC532的起始自放热温度均为150 ℃左右,4.4 V SC-NMC532在约150 ℃自放热率急剧上升。4.1 V、4.2 V和4.3 V。NMC622A在约215 ℃自放热速率显著提高,4.4 V NMC622A在约175 ℃显著自放热。4.1 V和4.2 V 622B显著自放热出现在150 ℃附近,而4.4 V 622B显著自放热则提前至140 ℃。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Fig. 5. SHR vs. temperature for different delithiated positive electrodes reacting with control electrolyte 1.09 mol/kg LiPF6 in EC: EMC (3:7) at (a) 4.1 V, (b) 4.2 V, (c) 4.3 V and (d) 4.4 V. The results for duplicate samples are given as a dashed line in each panel.

在4.1 V和4.2 V,NMC622A和NMC622B反应活性均低于SC-NMC532;在4.3 V和4.4 V,NMC622B反应活性高于SC-NMC532和NMC622A。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Fig. 6. SHR vs. temperature for delithiated (a) R532, (b) 622A and (c) 622B reacting with electrolyte 1.09 mol/kg LiPF6 in EC: EMC (3:7) containing additives (2% VC + 1% DTD for SC-NMC532, 2% VC + 1% MMDS for NMC622A and NMC622B) at different cut-off voltages. The results for duplicate samples are given as a dashed line in each panel.

图6为不同脱锂态三种NMC材料同含添加剂电解液在不同温度下的自放热率曲线。从4.1 V到4.4V,SC-NMC532、NMC-622A和NMC-622B三种材料的起始自放热温度不断降低,其中NMC-622A和NMC-622B尤为明显。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Fig. 7. SHR vs. temperature for the different delithiated positive electrodes reacting with electrolyte 1.09 mol/kg LiPF6 in EC: EMC (3:7) containing additives (2% VC + 1% DTD for SC-NMC532, 2% VC + 1% MMDS for NMC622A and NMC622B) at (a) 4.1 V, (b) 4.2 V, (c) 4.3 V and (d) 4.4 V. The results for duplicate samples are given as a dashed line in each panel.

在4.1 V和4.2 V,NMC622A和NMC622B的自放热曲线几乎一致,且在全温度范围内反应活性高于SC-NMC532。在4.3 V和4.4 V,NMC622B反应活性高于SC-NMC532和NMC622A。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Fig. 8. The temperature where the self-heating rate reaches 0.2 °C/min plotted versus the remaining lithium content, x, in Lix[NMC]O2. This compares all the materials at the same degree of delithiation. Results from Ma et al. [3] are included in Fig. 7b.如图8a所示,当锂含量高于0.25,三种材料中NMC622A化学稳定性最好;但在4.4 V(x=0.23),SC-NMC532化学稳定性更好。此外,添加剂的引入对三种正极材料的化学活性未产生显著影响。图8b对比了一系列NMC材料在不同脱锂态下同电解液的反应活性。NMC811“鹤立鸡群”, 简直不要太猛。

5. Ning Zhang, Jing Li, Hongyang Li, Aaron Liu, Que Huang, Lin Ma, Ying Li, and J. R. Dahn. Structural, Electrochemical and Thermal Properties of Nickel-rich LiNixMnyCozO2 Materials. Chemistry of Materials. DOI: 10.1021/acs.chemmater.8b03827.

链接:

https://pubs.acs.org/doi/10.1021/acs.chemmater.8b03827

要点:作者认为相较目前商业化应用的NMC622,NMC721、NMC631和NMC6.5:2.5:1所含Co降低了一半,但有着较好的克容量发挥和稳定性,值得后续考虑。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Table 4 Summary of the reversible and irreversible capacity of different NMC materials synthesized with Li/TM ratios of 1.05 and 1.07.

本工作合成了一系列NMC材料,旨在对比不同组分NMC电化学性能和材料热稳定性的差异。其中,系列材料首次循环的克容量信息和首效信息如表4所示。不难看出,随着Co含量的降低,不可逆容量和不可逆容量所占的比重不断提高。

 

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 6. Differential capacity as a function of voltage for the LiNi0.6Mn0.4-xCoxO2 (x=0.2, 0.1, 0) series (a1 to a3), the LiNi0.9-xMnxCo0.1O2 (x=0.25, 0.2, 0.1) series (b1 to b3) and the LiNi0.8Mn0.2-xCoxO2 (x=0.2, 0.1, 0) series (c1 to c3) at C/20. The voltage range is 3.0-4.4 V.

图6为不同脱锂态下LiNi0.6Mn0.4-xCoxO2 (x=0.2, 0.1, 0) (a1-a3)、LiNi0.9-xMnxCo0.1O2 (x=0.25, 0.2, 0.1) (b1 to b3)和LiNi0.8Mn0.2-xCoxO2 (x=0.2, 0.1, 0) (c1-c3)的dQ/dV曲线。文中对dQ/dV没有进行详细解读。值得注意的是NMC811在4.2 V有尖锐峰,而NMC622在4.2 V则没有。从NMC811、NMC721到NMC6.5:2.5:1、NMC631,随着Ni含量的降低、Mn含量的提高,4.2 V位置峰面积不断减小,表明循环稳定性不断提高。

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 7. Summary of the first cycle discharge capacity, irreversible capacity and percent irreversible capacity of the first cycle as a function of Co content (a1 to a3, c1 to c3) and Mn content (b1 to b3).

对于Ni60体系,当Ni含量一定时,随着Co含量的提高、Mn含量降低,NMC克容量有所提高,同时不可逆容量降低(图7a1-a3)。当Co含量一定时,随着Mn含量的提高、Ni含量降低,NMC克容量降低,同时不可逆容量升高(图7b1-b3)。对于Ni80体系,从NMC820、NMC811到NMC802,NMC802克容量最高,同时不可逆容量最低。

 

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 9. Self-heating rate as a function of temperature for delithiated NMC electrode samples reacting with electrolyte in accelerating rate calorimeter experiments. The samples were tested between 70 and 350 ℃. 1.2 M LiPF6 in EC:EMC (v/v 3:7) was used as the electrolyte. The electrodes were charged to 4.5 V vs. Li/Li+ or 4.4 V vs. graphite. Duplicate results were performed.

图9为四种NMC材料在电解液存在条件下的ARC测试结果。四种材料中NMC631热稳定性最好。

 

2014-2018年Jeff Dahn课题组应用ARC相关研究小结

Figure 10. Temperature where the self-heating rate first reached 0.2 K/min as a function of x in Lix(NMC)O2. Results for NMC631 and NMC721 (this work) are shown in (a), while results from Huang·and Ma·et al.5 are included in (b) at various degrees of de-lithiation.

作者对以往不同时期所测的NMC材料ARC结果进行了汇总(图10)。如图10a所示,在4.5 V,NMC631的稳定性远高于NMC721,且化学反应活性同4.2 V (x=0.32)和4.3 V (x=0.26)的NMC442相当,远低于NMC811(图10b)。

供稿丨深圳市清新电源研究院

部门丨媒体信息中心科技情报部

撰稿人丨敌法师

主编丨张哲旭


2014-2018年Jeff Dahn课题组应用ARC相关研究小结

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2014-2018年Jeff Dahn课题组应用ARC相关研究小结

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