Li-rich Mn-based cathode has the advantages of high specific capacity and low cost, which is expected to be the cathode material for the next generation of high-energy density Li-metal batteries. However, in practical applications, its charging cutoff voltage is 4.8 V versus Li, leading to the failure of electrolyte oxygenation decompositions, thereby deteriorating the interface between the cathode and electrolyte, making stable battery cycling challenging. A novel high-voltage electrolyte based on LiPF6 is developed, in which 1, 1, 2, 2-tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropylether (TTE) is used as a solvent to promote coordination. The experimental and characterization results show that many Li anions participate in the solvated structure of Li+ coordination after introducing appropriate TTE in the electrolyte, which can form a 5-nm-thick fluorine-rich cathode electrolyte interphase (CEI) on the surface of the cathode, stabilizing its interface and inhibiting the degradation of the cathode layer structure. The Li-rich Mn-based cathode Li-metal battery with the new electrolyte has an 83.9% capacity retention rate after 400 cycles with an average efficiency of 99.8% (0.5 C). The 1.25-Ah Li-rich Mn-based cathode Li-metal battery pouch cell can provide 370 Wh/kg mass-energy density at 0.04 C and still has an 80% capacity retention rate after 45 cycles at 0.08 C, showing a good application prospect. This study is helpful to promote the application of Li-rich Mn-based cathode and provide an experimental basis for the research and development of high energy density Li-metal batteries.
Keywords：lithium-rich manganese-based cathode
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high voltage
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electrolyte solvent
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cathode-electrolyte interphase
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layered structure
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lithium metal pouch cell
然而，传统碳酸酯类电解液，在富锂锰基正极高充电截止电压(4.8 V vs Li/Li+)下会发生剧烈的界面副反应，影响电池的循环性能。Cha等[3]报道的Li1.17Ni0.17Mn0.5Co0.17O2/Li半电池，应用传统碳酸酯类溶剂的电解液在0.5 C下恒流充放电，循环70次后电池容量跳水，表现出较差的电化学循环性能。Fan等[4]报道溶剂碳酸乙烯酯(EC)自4.3 V的对锂电位开始持续氧化分解；Zhang等[5]报道，除了高电压的氧化，富锂锰基正极的亲核超氧基团会攻击碳酸酯溶剂，加剧电解液的分解；Li等[6]还发现电解液分解产物中的HF会溶出富锂锰基正极中的过渡金属元素，破坏正极结构的稳定性。
一种广泛报道的策略，是通过向电解液中加入添加剂的方式提升电池的循环性能。Zheng等[7]引入1%的苯基乙烯基砜，它有更高的反应活性，在电解液中先于其余组分被氧化，并且在正极表面原位形成具有保护性的CEI稳定电池循环。Li等[6]使用添加剂LiBOB，消除电解液分解产物HF，提升电池的循环性能。然而，CEI随着循环的进行会经历溶解与重构，伴随着高反应活性的添加剂在正极表面持续反应而被耗尽，导致添加剂难以持续保护电解液组分[8]。降低电解液中溶剂组分的反应性是另一种可行的策略，最高占据分子轨道(Highest occupied molecular orbital，HOMO)能级低的砜类溶剂[9]、腈类溶剂[10-11]和氟化溶剂[12-13]等被用作电解液的助溶剂[14]，能够减少溶剂分解提升高电压正极的长循环性能；高锂盐浓度电解液形成的更多锂盐阴离子参与配位的接触离子对(Contact ion pairs，CIP)和聚集体(Aggregate，AGG)的溶剂化结构，与常规锂盐浓度电解液的溶剂分离离子对(solvent separates ion pairs，SSIP)溶剂化结构相比，不仅能促进有限溶剂与Li+高度配位降低溶剂分子的反应性[15-16]，Zheng等[17]还证实形成CIP和AGG的溶剂化结构可以促进锂盐阴离子参与成膜，有助于提升电极-电解液中间相的无机组分从而稳定界面。只是高锂盐浓度的电解液，通常具有较大黏度[18-19]，不利于实际应用。Ken等[20]向高浓度的砜基电解液中引入低黏度的TTE开发的局部高浓电解液(localized high-concentration electrolyte，LHCE)，不仅降低了电解液整体的粘度，非溶剂化的TTE引入后还能保留浓缩电解液的特质，能够实现高电压正极的循环稳定性。只是在设计LHCE配方时，通常需要一种高溶解度的锂盐如LiTFSI、LiFSI，而排除了常见的锂盐LiPF6、LiClO4、LiBOB等的应用。
Fig. 1
Properties of electrolyte F-based, F-TTE and F-2TTE: Raman spectra (a) From 680 to 780 cm–1,(b) From 880 to 980 cm–1; (c) Calculated HOMO levels of solvent molecule; (d) LSV curves; (e) Contact angle between electrolytes and separate and ionic conductivity of electrolytes; (f) Rate performance
Fig. 2
Cyclic performance of LRM||Li battery using electrolyte F-based, F-TTE and F-2TTE: (a) Charge-discharge curves at 0.1 C, (b) Discharge specific capacity in the cycling; Charge-discharge curves of battery using (c) F-based electrolyte, (d) F-TTE electrolyte
Fig. 3
XPS data of LRM cathode after 100 cycles in F-based and F-TTE electrolyte:(a) C1s; (b) O1s; (c) F1s; (d) Quantitative analysis of different elements
图4
LRM 正极循环200次后的TEM图像和对应的FFT图(a) F-基；(b) F-TTE；(c) 循环200次前后正极的XRD数据；(d) 据XRD数据计算的结构参数 I(003)/I(104)和 R 值
Fig. 4
TEM images and the corresponding FFT for LRM after 200 cycles in (a) F-based; (b) F-TTE; (c) The XRD data of the cathode before and after 200 cycles; (d) Structural parameters I(003)/I(104) and R value calculated by XRD data
Fig. 5
Morphology of lithium metal in batteries with F-based electrolyte (a)—(c) and F-TTE electrolyte cycling for 200 cycles (d)—(f); Nyquist impedance diagram of battery after (g) 1st cycle, (h) 100th cycle, (i) 200th cycle
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... 然而，传统碳酸酯类电解液，在富锂锰基正极高充电截止电压(4.8 V vs Li/Li+)下会发生剧烈的界面副反应，影响电池的循环性能.Cha等[3]报道的Li1.17Ni0.17Mn0.5Co0.17O2/Li半电池，应用传统碳酸酯类溶剂的电解液在0.5 C下恒流充放电，循环70次后电池容量跳水，表现出较差的电化学循环性能.Fan等[4]报道溶剂碳酸乙烯酯(EC)自4.3 V的对锂电位开始持续氧化分解；Zhang等[5]报道，除了高电压的氧化，富锂锰基正极的亲核超氧基团会攻击碳酸酯溶剂，加剧电解液的分解；Li等[6]还发现电解液分解产物中的HF会溶出富锂锰基正极中的过渡金属元素，破坏正极结构的稳定性. ...
... 然而，传统碳酸酯类电解液，在富锂锰基正极高充电截止电压(4.8 V vs Li/Li+)下会发生剧烈的界面副反应，影响电池的循环性能.Cha等[3]报道的Li1.17Ni0.17Mn0.5Co0.17O2/Li半电池，应用传统碳酸酯类溶剂的电解液在0.5 C下恒流充放电，循环70次后电池容量跳水，表现出较差的电化学循环性能.Fan等[4]报道溶剂碳酸乙烯酯(EC)自4.3 V的对锂电位开始持续氧化分解；Zhang等[5]报道，除了高电压的氧化，富锂锰基正极的亲核超氧基团会攻击碳酸酯溶剂，加剧电解液的分解；Li等[6]还发现电解液分解产物中的HF会溶出富锂锰基正极中的过渡金属元素，破坏正极结构的稳定性. ...
3
... 然而，传统碳酸酯类电解液，在富锂锰基正极高充电截止电压(4.8 V vs Li/Li+)下会发生剧烈的界面副反应，影响电池的循环性能.Cha等[3]报道的Li1.17Ni0.17Mn0.5Co0.17O2/Li半电池，应用传统碳酸酯类溶剂的电解液在0.5 C下恒流充放电，循环70次后电池容量跳水，表现出较差的电化学循环性能.Fan等[4]报道溶剂碳酸乙烯酯(EC)自4.3 V的对锂电位开始持续氧化分解；Zhang等[5]报道，除了高电压的氧化，富锂锰基正极的亲核超氧基团会攻击碳酸酯溶剂，加剧电解液的分解；Li等[6]还发现电解液分解产物中的HF会溶出富锂锰基正极中的过渡金属元素，破坏正极结构的稳定性. ...
... 然而，传统碳酸酯类电解液，在富锂锰基正极高充电截止电压(4.8 V vs Li/Li+)下会发生剧烈的界面副反应，影响电池的循环性能.Cha等[3]报道的Li1.17Ni0.17Mn0.5Co0.17O2/Li半电池，应用传统碳酸酯类溶剂的电解液在0.5 C下恒流充放电，循环70次后电池容量跳水，表现出较差的电化学循环性能.Fan等[4]报道溶剂碳酸乙烯酯(EC)自4.3 V的对锂电位开始持续氧化分解；Zhang等[5]报道，除了高电压的氧化，富锂锰基正极的亲核超氧基团会攻击碳酸酯溶剂，加剧电解液的分解；Li等[6]还发现电解液分解产物中的HF会溶出富锂锰基正极中的过渡金属元素，破坏正极结构的稳定性. ...
... 一种广泛报道的策略，是通过向电解液中加入添加剂的方式提升电池的循环性能.Zheng等[7]引入1%的苯基乙烯基砜，它有更高的反应活性，在电解液中先于其余组分被氧化，并且在正极表面原位形成具有保护性的CEI稳定电池循环.Li等[6]使用添加剂LiBOB，消除电解液分解产物HF，提升电池的循环性能.然而，CEI随着循环的进行会经历溶解与重构，伴随着高反应活性的添加剂在正极表面持续反应而被耗尽，导致添加剂难以持续保护电解液组分[8].降低电解液中溶剂组分的反应性是另一种可行的策略，最高占据分子轨道(Highest occupied molecular orbital，HOMO)能级低的砜类溶剂[9]、腈类溶剂[10-11]和氟化溶剂[12-13]等被用作电解液的助溶剂[14]，能够减少溶剂分解提升高电压正极的长循环性能；高锂盐浓度电解液形成的更多锂盐阴离子参与配位的接触离子对(Contact ion pairs，CIP)和聚集体(Aggregate，AGG)的溶剂化结构，与常规锂盐浓度电解液的溶剂分离离子对(solvent separates ion pairs，SSIP)溶剂化结构相比，不仅能促进有限溶剂与Li+高度配位降低溶剂分子的反应性[15-16]，Zheng等[17]还证实形成CIP和AGG的溶剂化结构可以促进锂盐阴离子参与成膜，有助于提升电极-电解液中间相的无机组分从而稳定界面.只是高锂盐浓度的电解液，通常具有较大黏度[18-19]，不利于实际应用.Ken等[20]向高浓度的砜基电解液中引入低黏度的TTE开发的局部高浓电解液(localized high-concentration electrolyte，LHCE)，不仅降低了电解液整体的粘度，非溶剂化的TTE引入后还能保留浓缩电解液的特质，能够实现高电压正极的循环稳定性.只是在设计LHCE配方时，通常需要一种高溶解度的锂盐如LiTFSI、LiFSI，而排除了常见的锂盐LiPF6、LiClO4、LiBOB等的应用. ...
