An overlooked issue for high-voltage Li-ion batteries: Suppressing the intercalation of anions into conductive carbon

Abstract

•The anion intercalation into carbon is an overlooked issue for high-voltage systems•A concentrated sulfolane-based electrolyte helps to prevent the anion intercalation•An extremely stable 5.2 V-class Li2CoPO4F/graphite full cell was achieved The energy stored in batteries is defined as the product of capacity and voltage. Because the capacity is reaching the theoretical limit of the Li-ion battery concept, increasing the voltage from the current 3.8 to 5 V is the major target to achieve high-energy densities. Various approaches have been taken to address the issues of 5 V Li-ion batteries, including the oxidative decomposition of the electrolyte at high voltages. However, their stable charge-discharge operation has not been achieved, suggesting the presence of an unknown yet essential issue that must be solved. Here, we unveil the intercalation of counter anion of an electrolyte into cathode conductive carbon as an overlooked critical issue. On this basis, we design a specific electrolyte that blocks the anion intercalation and demonstrate the unprecedented stable charge-discharge cycling of 5 V Li-ion batteries. This finding offers a fundamental basis for developing advanced Li-ion batteries with high-energy densities. High-voltage Li-ion batteries have been extensively studied to increase energy density of batteries. However, their cycling stability has remained poor, despite various strategies being proposed to overcome the issues of high-potential cathodes, such as electrolyte oxidation and transition metal dissolution. Herein, we report anion intercalation into the cathode conductive carbon as an overlooked yet critical issue. We propose a concentrated sulfolane (SL)-based electrolyte that prevents anion intercalation via two mechanisms: (1) by offering a high activation barrier to intercalation via its strong anion-Li+ interaction and (2) by forming a sulfur-containing, anion-blocking SL-derived interphase. This electrolyte, used with graphitized acetylene black, which is oxidatively stable but usually susceptible to anion intercalation, enables the stable operation of a Li2CoPO4F/graphite full cell (cut-off voltage = 5.2 V), with 93% capacity retention after 1,000 cycles and an average Coulombic efficiency of ≥99.9%. This is a pivotal strategy to enhance the reversibility of >5 V Li-ion batteries on a commercial level. High-voltage Li-ion batteries have been extensively studied to increase energy density of batteries. However, their cycling stability has remained poor, despite various strategies being proposed to overcome the issues of high-potential cathodes, such as electrolyte oxidation and transition metal dissolution. Herein, we report anion intercalation into the cathode conductive carbon as an overlooked yet critical issue. We propose a concentrated sulfolane (SL)-based electrolyte that prevents anion intercalation via two mechanisms: (1) by offering a high activation barrier to intercalation via its strong anion-Li+ interaction and (2) by forming a sulfur-containing, anion-blocking SL-derived interphase. This electrolyte, used with graphitized acetylene black, which is oxidatively stable but usually susceptible to anion intercalation, enables the stable operation of a Li2CoPO4F/graphite full cell (cut-off voltage = 5.2 V), with 93% capacity retention after 1,000 cycles and an average Coulombic efficiency of ≥99.9%. This is a pivotal strategy to enhance the reversibility of >5 V Li-ion batteries on a commercial level. Next-generation Li-ion batteries with high-energy densities are crucial for the development of advanced electrical devices, such as mobile phones and electric vehicles.1Armand M. Tarascon J.M. 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To suppress anion intercalation, we used a concentrated LiBF4− electrolyte in which the BF4− anions were intensely coordinated to Li+. This offers a high activation barrier to anion intercalation, because the BF4− anions have to first dissociate from Li+. We also adopted sulfolane (SL) as an electrolyte solvent to form a sulfur-containing, anion-blocking CEI on the conductive carbon. By utilizing a concentrated LiBF4-SL-based electrolyte in combination with graphitized acetylene black, which is oxidatively stable but is not suitable for traditional electrolytes because of its high susceptibility to anion intercalation, a Li2CoPO4F/graphite full cell with an extremely stable cycling (93% capacity retention after 1,000 cycles) was successfully achieved with an upper cut-off of 5.2 V at a slow 0.5 C-rate. This cell performance was far better than that of full cells operated at >5 V in previous publications (Table S1). The anion intercalation behavior is dependent on the combination and concentration of salts and solvents in the electrolyte,34Wang M. Tang Y. A review on the features and progress of dual-ion batteries.Adv. Energy Mater. 2018; 8: 1703320Crossref Scopus (219) Google Scholar as well as the graphitization degree of the carbon.19Ko S. Yamada Y. Lander L. Yamada A. Stability of conductive carbon additives in 5 V-class Li-ion batteries.Carbon. 2020; 158: 766-771Crossref Scopus (10) Google Scholar,25Heckmann A. Fromm O. Rodehorst U. Münster P. Winter M. Placke T. New insights into electrochemical anion intercalation into carbonaceous materials for dual-ion batteries: impact of the graphitization degree.Carbon. 2018; 131: 201-212Crossref Scopus (48) Google Scholar,26Qi X. Blizanac B. DuPasquier A. Meister P. Placke T. Oljaca M. Li J. Winter M. Investigation of PF6- and TFSI- anion intercalation into graphitized carbon blacks and its influence on high voltage lithium ion batteries.Phys. Chem. 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Solid-State Lett. 2007; 10: A74-A76Crossref Scopus (142) Google Scholar As shown in Figures 2A and S1, we performed linear sweep voltammetry (LSV) using graphite electrodes in SL-based electrolytes with a different concentration, whose basic physicochemical properties are shown in Table S2; Figure S2, and a commercial 1.0 M LiPF6/ethylene carbonate (EC):dimethyl carbonate (DMC) (1:1 v/v) electrolyte. The onset potential of anion intercalation was slightly upshifted in 1.0 M LiBF4/SL (4.78 V versus Li/Li+, Figure S1) and further upshifted in the nearly saturated 5.8 M LiBF4/SL (4.90 V versus Li/Li+, Figure 2A) compared with that in the commercial electrolyte (4.59 V versus Li/Li+, Figure 2A). The suppressed anion intercalation in the 5.8 M LiBF4/SL was further supported by the in situ Raman spectra recorded during galvanostatic cycling of the graphite electrodes (inset of Figure 2A). To shed more light on the effect of the electrolyte on anion intercalation, cyclic voltammetry (CV) was performed using graphite electrodes in three-electrode cells (Figure S3A). Generally, Li metal is used as a quasi-reference electrode, but its potential is dependent on the electrolyte composition with various Li+ activities and solvation energies.35Gagne R.R. Koval C.A. Lisensky G.C. Ferrocene as an internal standard for electrochemical measurements.Inorg. Chem. 1980; 19: 2854-2855Crossref Scopus (1026) Google Scholar,36Mozhzhukhina N. Calvo E.J. Perspective—the correct assessment of standard potentials of reference electrodes in non-aqueous solution.J. Electrochem. Soc. 2017; 164: A2295-A2297Crossref Scopus (25) Google Scholar Thus, the potential of the Li metal was calibrated to that of ferrocene (Fc/Fc+) to produce a true reference electrode with solvent-independent redox potential (Figure S4). 35Gagne R.R. Koval C.A. Lisensky G.C. Ferrocene as an internal standard for electrochemical measurements.Inorg. Chem. 1980; 19: 2854-2855Crossref Scopus (1026) Google Scholar,36Mozhzhukhina N. Calvo E.J. Perspective—the correct assessment of standard potentials of reference electrodes in non-aqueous solution.J. Electrochem. Soc. 2017; 164: A2295-A2297Crossref Scopus (25) Google Scholar As shown in Figure S3B, the difference in the intercalation potentials between the 1.0 M LiPF6/EC:DMC (1:1 v/v) (1.34 V versus Fc/Fc+) and 1.0 M LiBF4/SL (1.40 V versus Fc/Fc+) was insignificant. In contrast, the intercalation potential was dramatically upshifted in the 5.8 M LiBF4/SL (1.72 V versus Fc/Fc+). This upshift in potential contradicts the expectation of a decrease in potential based on its increased anion activity in the Nernst equation, suggesting the presence of a kinetic factor. Electrochemical impedance spectroscopy (EIS) analysis demonstrated that the interfacial resistance of the anion intercalation into graphite was significantly increased at high salt concentrations (Figures 2B and S5A–S5C). In particular, the activation energy of the interfacial anion transfer was 59 kJ mol−1 in the 5.8 M LiBF4/SL, which is nearly four times higher than that in 1.0 M LiPF6/EC:DMC (15 kJ mol−1) and 1.0 M LiBF4/SL (15 kJ mol−1) (Figures 2C and S5D). These results indicate that a high salt concentration retards anion intercalation (the role of the SL solvent is discussed in next section). In addition, LSV and electrochemical floating tests were performed on conductive carbon powders (acetylene black) with different degrees of graphitization (Figures 3 and S6). In the 1.0 M LiPF6/EC:DMC, the anodic current in the LSV and leakage current at 5.2 V were much higher on graphitized acetylene black (Figure 3) than on amorphous acetylene black (Figure S6). This is because anion intercalation actively occurs with increasing graphitization degree.19Ko S. Yamada Y. Lander L. Yamada A. Stability of conductive carbon additives in 5 V-class Li-ion batteries.Carbon. 2020; 158: 766-771Crossref Scopus (10) Google Scholar,25Heckmann A. Fromm O. Rodehorst U. Münster P. Winter M. Placke T. New insights into electrochemical anion intercalation into carbonaceous materials for dual-ion batteries: impact of the graphitization degree.Carbon. 2018; 131: 201-212Crossref Scopus (48) Google Scholar,26Qi X. Blizanac B. DuPasquier A. Meister P. Placke T. Oljaca M. Li J. Winter M. Investigation of PF6- and TFSI- anion intercalation into graphitized carbon blacks and its influence on high voltage lithium ion batteries.Phys. Chem. Chem. Phys. 2014; 16: 25306-25313Crossref PubMed Google Scholar,28Ishihara T. Koga M. Matsumoto H. Yoshio M. Electrochemical intercalation of hexafluorophosphate anion into various carbons for cathode of dual-carbon rechargeable battery.Electrochem. Solid-State Lett. 2007; 10: A74-A76Crossref Scopus (142) Google Scholar In contrast, 5.8 M LiBF4/SL showed the opposite trend and a much lower current, indicating that the anion intercalation was successfully suppressed. This was further proven by the galvanostatic cycling tests of carbon electrodes (Figure S7) in which irreversible charges (parasitic capacities) occurred to the lowest degree on graphitized acetylene black in the 5.8 M LiBF4/SL. Importantly, these results were reflected in the Li2CoPO4F/Li half-cell tests (Figures S8 and S9). The combination of 5.8 M LiBF4/SL and graphitized acetylene black enabled the stable cycling of Li2CoPO4F, with the highest Coulombic efficiency (and lowest leakage current). Overall, suppressing the anion intercalation, which strongly influences the stability of conductive carbon, enhances the reversibility of high-potential cathodes. To clarify the mechanism behind the retarded anion intercalation in the 5.8 M LiBF4/SL electrolyte, its solution structure was analyzed via Raman spectroscopy. As shown in Figure 4A, the Raman band of BF4− anions gradually upshifted from 767 to 777 cm−1 with increasing salt concentration, suggesting more extensive ion-pairing from solvent-separated ion pairs (SSIPs; BF4− anions not directly coordinated with Li+) to contact ion pairs (CIPs; one BF4− anion coordinated with one Li+ ion) and aggregate clusters (AGGs; BF4− anions coordinated with two or more Li+ ions)37Seo D.M. Boyle P.D. Allen J.L. Han S.-D. Jónsson E. Johansson P. Henderson W.A. Solvate structures and computational/spectroscopic characterization of LiBF4 electrolytes.J. Phys. Chem. 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Krachkovskiy S.A. et al.Lithium polyacrylate (LiPAA) as an advanced binder and a passivating agent for high-voltage Li-Ion batteries.Adv. Energy Mater. 2015; 5: 1501008Crossref Scopus (138) Google Scholar,17Saneifar H. Delaporte N. Zaghib K. Bélanger D. Functionalization of the carbon additive of a high-voltage Li-ion cathode.J. Mater. Chem. A. 2019; 7: 1585-1597Crossref Google Scholar,42Han X. Xu G. Zhang Z. Du X. Han P. Zhou X. Cui G. Chen L. An in situ interface reinforcement strategy achieving long cycle performance of dual-ion batteries.Adv. Energy Mater. 2019; 9: 1804022Crossref Scopus (53) Google Scholar To ensure the high reversibility of the graphite anode, FEC, which supports the efficient formation of a solid electrolyte interphase (SEI) layer, was introduced as a co-solvent in the 5.8 M LiBF4/SL electrolyte. The developed 6.6 M LiBF4/SL:FEC (9:1 n/n) electrolyte improved the Coulombic efficiency and cycling performance of the graphite anodes (Figure S13) while maintaining the merits of the 5.8 M LiBF4/SL electrolyte, in that it suppressed the anion intercalation (Figures S14–S17), exhibited high oxidation stability on Pt (Figure S18), and enabled the stable cycling of Li2CoPO4F/Li half cells (Figures S19 and S20). The long-term cycling performance of 4.8 V Li2CoPO4F/graphite full cells is shown in Figure 5. The extremely high cycling stability of the full cell up to a cut-off of 5.2 V was achieved with the combination of a 6.6 M LiBF4/SL:FEC (9:1 n/n) electrolyte and graphitized acetylene black carbon material. The cell retained 93% of the initial discharge capacity after 1,000 cycles (only 0.007% capacity loss per cycle) with an average Coulombic efficiency of ≥99.9%, which is far higher than those obtained with the state-of-the-art commercial 1.0 M LiPF6/EC:DMC (1:1 v/v) electrolyte (∼60% capacity retention after 350 cycles with a poor Coulombic efficiency of <97%) and other electrolytes (Figure S21). This unprecedented cycling performance is attributed to the suppression of anion intercalation into the conductive carbon (Figures S14–S17 and S22–S23), enhanced oxidation stability (Figure S18), and also a reduction in the transition metal dissolution from the Li2CoPO4F cathode (Figure S24). These attributes are unique to salt-concentrated SL electrolytes. The intercalation of anions into conductive carbon is a major issue that has hindered the realization of Li-ion batteries with commercial-level reversibility that operate at >5 V. To solve this, we proposed the use of concentrated LiBF4/SL-based electrolytes, which offer two benefits: (1) a high activation energy of BF4− anion intercalation owing to the strong BF4−-Li+ interactions, which is unique to high salt-concentrated electrolytes; and (2) formation of an SL-derived functional CEI that acts as an anion-blocking layer. These concentrated LiBF4/SL-based electrolytes can be used with oxidatively stable, graphitized acetylene black without activating BF4− intercalation up to 5.2 V. As a result, the Li2CoPO4F/graphite full cell achieved unprecedented cycling performance of up to a cut-off of 5.2 V, with 93% capacity retention after 1,000 cycles and an average Coulombic efficiency of ≥99.9%. This novel strategy for protecting the carbon material at high potentials of >5 V will be an essential step toward the commercialization of 5 V Li-ion batteries.